HWB 11a 11b prep par rev

EXAM 3

[Endocrine system basics] What is the endocrine system, what are hormones, and how do hormones act at target tissues?

The endocrine system is several ductless glands that secrete chemical messengers (hormones) directly into the bloodstream. At target tissues hormones interact with specific high-affinity receptors: located on the plasma membrane for protein (peptide) hormones or amines, and intracellular (nuclear, cytosolic) for thyroid and steroid hormones.

[Endocrine system basics] In which four broad physiological areas do hormones function?

Maintenance of internal environment, energy balance, growth and development, and reproduction.

[Endocrine system basics] How is endocrine secretion regulated, and what is the advantage of an endocrine axis?

Through feedback loop control systems tuned to set-points, which may be modified by circadian rhythm, seasonal cycles, age and other influences such as stress. Many hormones function as part of a cascade (endocrine axis: hypothalamus-pituitary-target gland), which permits signal amplification, flexibility of response to a variety of physiological stimuli and fine regulation of the end hormone product.

[Endocrine system basics] What is negative feedback in the endocrine axis, and what are the long and short feedback loops (Fig 45.1)?

Negative feedback: the release of hormone leads to events that inhibit further secretion. At several points in the axis this keeps plasma hormone levels close to the set-point. Long feedback loop: the peripheral hormone inhibits the hypothalamus (indirect) and the anterior pituitary (direct). Short feedback loop: the tropic hormone of the anterior pituitary inhibits the hypothalamus. Secretion of hypothalamic releasing hormones may also be modulated by neuronal input from higher levels of the brain.

[Hypothalamus and pituitary] Describe the anatomy of the hypothalamic-pituitary complex (infundibulum, hypophysis, sella turcica, optic chiasm) and why a lesion can cause visual field defects.

Nuclei (collections of neuronal cell bodies) in the hypothalamus are involved in endocrine function. A funnel-shaped stalk, the infundibulum, extends from the floor of the hypothalamus to the pituitary gland (hypophysis), which lies in a bony cavity (sella turcica) in the sphenoid bone. The optic chiasm is an important anatomical relation, so any expanding lesion of the pituitary or hypothalamus can present with visual field defects.

[Hypothalamus and pituitary] What are the two lobes of the pituitary gland and what is their developmental origin?

Anterior lobe (adenohypophysis): derives from the ectodermal Rathke's pouch, an extension of the roof of the primitive oral cavity. Posterior lobe (neurohypophysis): forms from a neural down-growth of the di-encephalon. The two tissues migrate and fuse to give rise to the pituitary gland.

[Hypothalamus and pituitary] How do hypothalamic releasing hormones reach the anterior pituitary, and what happens if the pituitary stalk is damaged?

Peptide hormones are synthesised in the cell bodies of small parvocellular neurones and released from axonal endings into capillary loops at the median eminence. They are transported via the hypothalamic-hypophyseal portal vessels to the capillary plexus within the anterior pituitary, where they stimulate or inhibit the release of hormones of specific cell types. Damage to the pituitary stalk (physical trauma, surgery, disease) impairing blood flow to the pituitary can lead to disrupted control of anterior pituitary hormone release.

[Hypothalamus and pituitary] Give the hypothalamic releasing hormones, the anterior pituitary hormone each controls, the peripheral target and the effect (Fig 45.2).

GnRH (+): LH and FSH, act on the gonads (sex hormones): reproduction. GHRH (+) and somatostatin (-): GH, acts on the liver (IGF-1): growth. TRH (+): TSH, acts on the thyroid (thyroid hormones): metabolism. Dopamine (-): prolactin, acts on breast tissue: lactation. CRH (+): ACTH, acts on the adrenal cortex (cortisol): stress. Prolactin is under tonic inhibitory control by the hypothalamus, unlike the other anterior pituitary hormones.

[Hypothalamus and pituitary] Which hypothalamic nuclei produce which hormones (Fig 45.3)?

Paraventricular nucleus: CRH, TRH. Medial preoptic nucleus: GnRH. Periventricular nucleus: somatostatin, TRH. Arcuate nucleus: GHRH, dopamine. Supraoptic nucleus (with the paraventricular nucleus): vasopressin and oxytocin (magnocellular neurones).

[Hypothalamus and pituitary] Which five secretory cell types make up the anterior pituitary? Give the hormone, percentage and structure of each (Table 45.1).

Acidophil cells: somatotroph (~50%) makes growth hormone (GH); lactotroph (~10-25%) makes prolactin (PRL); both are single-chain polypeptides with disulphide bonds. Basophil cells: gonadotroph (~20%) makes FSH and LH (glycoproteins); corticotroph (~10%) makes ACTH (peptide); thyrotroph (5-10%) makes TSH (glycoprotein). Anterior pituitary hormones act on peripheral endocrine glands to stimulate hormone secretion (tropic effect) or on non-endocrine tissue for direct effects (e.g. prolactin on mammary tissue).

[Hypothalamus and pituitary] Where are vasopressin and oxytocin made, how do they reach the posterior pituitary, and what triggers their release?

They are synthesised in the cell bodies of magnocellular neurones in the supraoptic and paraventricular nuclei (different subsets of neurones make either vasopressin or oxytocin). They are transported down the neuronal axons (hypothalamic-hypophyseal neural tract) to the posterior pituitary, where they are stored. Release is stimulated by neural signals (neuroendocrine reflex).

[Hypothalamus and pituitary] What regulates vasopressin (AVP / ADH) release and what are its main effects?

Release is regulated by plasma osmolality (detected by hypothalamic osmoreceptors) and by blood pressure/volume (detected by baroreceptors), with neural signals sent to hypothalamic nuclei. Main effects in humans: increased renal water reabsorption (V2-receptors) and vasoconstriction (V1-receptors).

[Hypothalamus and pituitary] What triggers oxytocin release, what does it do, and why is it special?

Suckling or stimulation of the cervix (copulation or parturition) generates neural signals relayed to oxytocinergic neurones in the hypothalamus; action potentials to the posterior pituitary release oxytocin into the blood. Oxytocin stimulates contraction of the myoepithelial cells that eject milk from the breast and contraction of the uterus (myometrium). It is a rare example of positive feedback.

[Hypothalamus and pituitary] What causes endocrine disorders, how are they diagnosed, and what are primary, secondary and tertiary disorders?

Causes: hormone deficiency, hormone excess or hormone resistance. Baseline hormone levels and the feedback relationships within an endocrine axis form the basis of diagnostic (dynamic or provocative) testing to determine where the defect lies. Defects at the peripheral endocrine gland = primary; at the anterior pituitary = secondary; at the hypothalamus = tertiary.

[Hypothalamus and pituitary] Describe pituitary disease: the most common cause, tumour types and how adenomas cause problems.

Most common cause is pituitary tumours: ~10% of all intracranial neoplasms, over 99% are benign (non-invasive) adenomas, either microadenomas (less than 10 mm) or macroadenomas. Adenomas cause problems by hyper-secretion of a pituitary hormone, inadequate production of the other pituitary hormones, and local effects on related structures (e.g. headaches, visual disturbances). ~75% are endocrinologically active; prolactin-producing adenomas (prolactinomas) are the most common (~40%). Growth hormone-producing adenomas produce acromegaly (adult).

[Hypothalamus and pituitary] What is hypopituitarism and pan-hypopituitarism?

Hypopituitarism is an underproduction of pituitary hormones. Pan-hypopituitarism is when all the pituitary hormones are underproduced. Clinically, hypopituitarism usually varies depending on the dominant hormone that is deficient.

[Endocrine pancreas] Why is glucose homeostasis important and which three coordinated processes does it depend on?

Glucose is an essential energy substrate for many tissues and narrow control of blood glucose is central for a constant glucose supply to the brain. It depends on: insulin secretion by the pancreas, hepatic glucose output, and glucose uptake by splanchnic (liver and gut) and peripheral tissues (muscle and fat). This involves hormonal and neural mechanisms in the CNS, the autonomic nervous system and the endocrine pancreas.

[Endocrine pancreas] Describe the islets of Langerhans (number, size, share of pancreas, blood flow, innervation, cell types and distribution).

About one million islets (diameter 50-300 μm), scattered throughout the pancreas, ~1-2% of total pancreatic mass, highly vascularised (~10% of pancreatic blood flow) and richly innervated by the autonomic nervous system and the GI neuroendocrine system. At least five peptide hormone-producing cells: alpha, beta, delta, PP, epsilon (A, B, D, F, epsilon). Most islets are mainly insulin-secreting beta-cells with fewer alpha- and delta-cells. Islets in the posterior portion of the head of the pancreas are pancreatic polypeptide-rich (PP-cells 80%). Ghrelin-secreting epsilon-cells are described in fetal islets and become rare soon after birth. Islet cells influence each other by paracrine (adjacent cells) and autocrine (act on the original secretory cell) communication. Islets also contain agents (e.g. neuropeptide Y, galanin) that influence exocrine function.

[Endocrine pancreas] Give the cell types of the islet with % of islet, hormone, function and paracrine effect (Table 46.1).

Alpha (A), 30-40%: glucagon; counter-regulatory hormone to insulin, increases blood glucose; paracrine (+) insulin secretion. Beta (B), 50-60%: insulin and amylin; lowers blood glucose (amylin delays gastric emptying); paracrine (-) glucagon secretion. Delta (D), 5-10%: somatostatin; inhibitory effects on digestive processes, islet hormone secretions?; paracrine (-) islet cell secretions. PP (F), 1-5%: pancreatic polypeptide; inhibition of exocrine secretion?; paracrine (-) islet cell secretions. Epsilon, <1%: ghrelin; promotes cell growth and maturation in the developing pancreas?; paracrine (-) insulin secretion.

[Endocrine pancreas] Describe the structure of insulin and its plasma half-life.

Insulin is a 51-amino acid peptide hormone, first synthesised as the prohormone proinsulin; the internal C-peptide is cleaved to give mature insulin, which has an A-chain (21 amino acids) connected to a B-chain (30 amino acids) by two disulfide bonds. Like other peptide hormones it has a short plasma half-life of several minutes.

[Endocrine pancreas] What regulates insulin secretion (Fig 46.1)?

The main physiological regulator is plasma glucose, influenced by other nutrients (amino acids, fatty acids), gastrointestinal hormones (incretins, e.g. glucagon-like peptide 1, GLP-1) and nervous input (muscarinic receptors on the parasympathetic nerves; alpha2-adrenoceptors on the sympathetic nerves and adrenaline).

[Endocrine pancreas] How does insulin act at target tissues, and how does it lower blood glucose?

Insulin binding to its receptor causes autophosphorylation via the receptor's intrinsic tyrosine kinase activity, starting a series of interactions with docking proteins and a cascade of phosphorylation/dephosphorylation reactions. Insulin lowers blood glucose by: increasing glucose uptake in adipose and skeletal muscle; increasing glycogen synthesis and inhibiting glycogenolysis; increasing protein synthesis and inhibiting protein breakdown; increasing lipogenesis and inhibiting lipolysis; inhibiting gluconeogenesis. Other effects: uptake of cellular K+ ions (via Na+/K+ ATPase), growth regulation and control of vascular function.

[Endocrine pancreas] What is glucagon, what does it do and what regulates its secretion?

A 29-amino acid peptide and the primary counter-regulatory hormone to insulin. It increases blood glucose by stimulating hepatic glucose output (elevated glycogenolysis and gluconeogenesis) and lipolysis. It is rapidly secreted when plasma glucose falls and inhibited by elevated glucose. Other stimuli: amino acids (e.g. a protein-rich meal) and activation of both sympathetic and parasympathetic nervous input.

[Endocrine pancreas] What is somatostatin in the pancreas (forms, release, suggested roles)?

It exists as a 14-amino acid and a 28-amino acid peptide. Its release is glucose dependent. Its physiological role is unclear; suggestions: regulation of nutrient flow via control of digestion (endocrine) and a tonic inhibitory influence on insulin and glucagon secretion (paracrine).

[Diabetes mellitus] What is diabetes mellitus (DM) and what are its signs and symptoms?

A metabolic disorder characterised by chronic hyperglycaemia. Signs and symptoms: glucosuria (plasma glucose exceeds the transport maximum for renal absorption, so glucose appears in the urine), polyuria and thirst (due to osmotic diuresis), and fatigue/malaise.

[Diabetes mellitus] What are the WHO diagnostic criteria for DM (Table 46.2)?

The patient has symptoms and one of the following: fasting plasma glucose >= 7.0 mmol/L (after an 8-hour overnight fast); random plasma glucose >= 11.1 mmol/L; plasma glucose > 11.1 mmol/L 2 hours after 75 g glucose in an oral glucose tolerance test (OGTT); HbA1c > 6.5% or 48 mmol/mol. If the patient is asymptomatic, two independent plasma glucose measurements in the diabetes range on separate days are required. HbA1c (glycated haemoglobin) correlates with the average blood glucose level over the last 8-12 weeks (life span of red blood cells).

[Diabetes mellitus] What are impaired glucose tolerance and impaired fasting glycaemia (Table 46.2)?

Impaired glucose tolerance: fasting

[Diabetes mellitus] Describe type 1 diabetes (T1DM): cause, risk factors, antibodies, complications and treatment.

An autoimmune condition causing specific destruction of the islet beta-cells, with absolute insulin deficiency; ~70-90% beta-cell loss occurs before clinical disease. Genetic factors (~30% susceptibility, association with HLA haplotypes DR3 and DR4) and environmental triggers (such as viral, toxins) are important. Autoantibodies to glutamic acid decarboxylase (GAD), insulin and insulinoma-associated protein (IA-2) may be detected. Lack of insulin action can lead to excessive hepatic fatty acid beta-oxidation with abnormal production of ketone bodies, causing metabolic acidosis (ketoacidosis). Patients require exogenous insulin. Accounts for 5-15% of all DM.

[Diabetes mellitus] Describe type 2 diabetes (T2DM): cause, mechanisms and share of DM.

A heterogeneous condition caused by impaired insulin secretion, decreased insulin sensitivity in target tissues (insulin resistance), or both. Strong genetic component plus risk factors such as obesity and sedentary lifestyle. Mechanisms of insulin resistance are not fully established: defects in insulin signalling and abnormal secretion of adipose-derived factors (adipokines) such as resistin and TNF-alpha may be involved. Beta-cell function progressively declines as the elevated insulin demands exhaust beta-cell secretory capacity; the resulting hyperglycaemia then causes the symptoms of DM. Accounts for 85-95% of cases of DM.

[Diabetes mellitus] How is T2DM treated (lifestyle, drugs that increase beta-cell output, drugs that improve insulin action, incretin-based drugs, advanced disease)?

Primary treatment for insulin resistance: diet and lifestyle: exercise (increases insulin sensitivity of peripheral tissues) and weight loss (if overweight/obese). Drugs to increase beta-cell output: sulphonylureas (inhibitors of the ATP-sensitive K-channel on beta-cells) and meglitinides (post-prandial glucose regulators). Drugs to improve insulin action: metformin (a biguanide; activator of AMP kinase) and glitazones (thiazolidinediones; PPAR-gamma agonists). Incretin-based medicines (GLP-1 analogues and dipeptidyl peptidase IV [DPP-IV] inhibitors, gliptins) increase insulin release, promote satiety and delay gastric emptying. In advanced T2DM, where the insulin secretory response is severely compromised, exogenous insulin is required.

[Diabetes mellitus] What are the chronic complications of diabetes?

Microvascular: retinopathy, nephropathy and neuropathy (neuropathy is specific to patients with DM, due to damage of small vessels (capillary pathology) by hyperglycaemia (e.g. sorbitol) and glycation of proteins). Macrovascular: ischaemic heart disease, stroke and peripheral vascular disease, due to accelerated atherosclerosis.

[Thyroid gland] Where is the thyroid gland and what is its anatomy?

Located anterior to the cricoid cartilage in the anterior neck. Two lobes are connected by the isthmus, which lies over the second and third tracheal rings. It is a very vascular organ (in severe hyperplasia greater blood flow may become audible as a bruit). Two pairs of parathyroid glands are on the posterior surface of the thyroid.

[Thyroid gland] Describe the histology of the thyroid and parathyroid glands.

Thyroid: many follicles, each formed by a single layer of cuboidal epithelial cells (follicular cells) that secrete into the follicle interior; the lumen is filled with a proteinaceous, viscous material called colloid, which stores thyroid hormones on the glycoprotein thyroglobulin. Larger parafollicular (C) cells are scattered around the follicles and produce calcitonin. Parathyroid: parathyroid hormone (PTH) is secreted by chief cells; the function of oxyphil cells is unknown.

[Thyroid gland] What do PTH and calcitonin do, and what controls their release?

PTH: secretion is stimulated by low plasma calcium. It raises plasma calcium by stimulating calcium resorption of bone, stimulating calcium uptake from the GI tract and enhancing renal excretion of phosphate. PTH is required for renal 'activation' of vitamin D into 1,25-dihydroxycholecalciferol, which promotes calcium reabsorption from the gut. Calcitonin: released in response to high plasma calcium; lowers plasma calcium by inhibiting bone resorption and inhibiting reabsorption in the kidney.

[Thyroid gland] Describe the synthesis of thyroid hormones step by step (iodide trapping, TPO, thyroglobulin, MIT/DIT, T3/T4, storage).

Thyroid hormones are the only hormones that require an essential trace element, iodine. (1) The follicular cell actively accumulates iodide (I-) via the Na+-I- symporter (driven by the inward Na+ gradient) on the basement membrane = iodide trapping. (2) At the apical (luminal) membrane, I- is transported into the follicle lumen by pendrin. (3) I- is oxidised into an active intermediate by thyroid peroxidase (TPO). (4) TPO also catalyses iodination of tyrosine residues (organification) on thyroglobulin (a glycoprotein rich in tyrosine) to give mono-iodotyrosine (MIT) and di-iodotyrosine (DIT). (5) TPO catalyses coupling: two DIT form thyroxine (T4); one MIT + one DIT form tri-iodothyronine (T3). (6) Hormones are stored as part of thyroglobulin (colloid), enough to supply the body's normal requirements for 2-3 months.

[Thyroid gland] How are thyroid hormones released, and what is the ratio of T4 to T3 to rT3?

Small portions of colloid are taken up by endocytosis; the vesicles fuse with lysosomes, whose proteases digest thyroglobulin to release T4 and T3, which then enter the bloodstream (exact mechanism unclear). ~95% of secreted thyroid hormone is T4, the remainder is T3, with negligible amounts of reverse T3 (rT3, formed by coupling of DIT with MIT).

[Thyroid gland] How are thyroid hormones transported and activated in the blood and tissues?

As lipophilic molecules they are poorly soluble in plasma and 99.9% bound to plasma-binding proteins (thyroxine-binding globulin, transthyretin and albumin). De-iodinase enzymes in plasma and peripheral tissues metabolise T4 to the more biologically active T3 (about 4-5 times more potent) and the inactive rT3; so T4 acts as a pro-hormone to T3.

[Thyroid gland] How is thyroid function regulated (hypothalamic-pituitary-thyroid axis, TSH effects, other stimuli, goitre)?

Regulated by thyroid-stimulating hormone (TSH), with thyroid hormones exerting negative feedback on the hypothalamic-pituitary-thyroid axis (TRH -> TSH -> T3, T4). Other stimuli: inhibitory (stress, somatostatin, glucocorticoids) and stimulatory (cold exposure in infants). TSH stimulates the synthesis and release of thyroid hormones, including thyroglobulin synthesis. TSH also has a trophic (to nourish, promote growth) effect: it stimulates thyroid growth with hyperplasia and hypertrophy of follicular cells. Excess plasma TSH can lead to a goitre (enlarged thyroid gland).

[Thyroid gland] How do thyroid hormones act in cells, and what are their metabolic and cardiovascular effects?

They diffuse across the plasma membrane and interact with specific nuclear receptors (members of the superfamily of ligand-dependent transcription factors), altering gene expression. Non-genomic actions: direct effects on Na+/K+-ATPase and Ca2+-ATPase. Metabolism: increase basal metabolic rate, oxygen consumption and heat production; stimulate anabolic and catabolic processes of protein, fat and carbohydrate metabolism. Cardiovascular: increase heart rate, cardiac output and blood pressure; also act synergistically with catecholamines by upregulating beta-adrenoceptors.

[Thyroid gland] What is the role of thyroid hormones in growth and development?

Essential for normal development and maturation of the brain, and for skeletal growth and bone maturation. They promote growth hormone release from the pituitary and act in a permissive manner on the actions of growth hormone. Lack of thyroid hormone in the developing newborn leads to cretinism (stunted growth and mental retardation).

[Thyroid gland] Describe hypothyroidism: frequency, causes, Hashimoto's thyroiditis, symptoms and treatment.

Thyroid hormone deficiency, estimated to affect 1-2% of adults in their lifetime. The most common cause worldwide is iodine deficiency; it may also follow radioactive iodine therapy or thyroid surgery. Primary failure is usually due to autoimmune destruction or attack of the thyroid; Hashimoto's thyroiditis accounts for 90% of all hypothyroidism and is a slowly developing chronic autoimmune disease with an immune reaction to thyroglobulin and other thyroid components. Symptoms: lethargy, cold intolerance, weight gain, constipation. Treatment: synthetic thyroid hormones (such as levo-thyroxine).

[Thyroid gland] Describe hyperthyroidism (thyrotoxicosis): features, Graves' disease and treatment options.

Elevated plasma T3 and T4, with sweating, tremor, tachycardia and weight loss. The most common cause is Graves' disease: autoantibodies against the TSH receptor (thyroid-stimulating immunoglobulins, TSI) activate the TSH receptor and stimulate the thyroid to produce and secrete more T3 and T4. Patients present with a goitre. Lymphocytic infiltration and fluid deposition in the orbital soft tissue cause protrusion of the eyeballs (proptosis, exophthalmos). Treatment: beta-blockers (rapid sympathetic relief of beta-adrenoceptor-mediated effects, e.g. tremor, tachycardia), antithyroid drugs (e.g. carbimazole, which inhibits TPO activity and so thyroid hormone synthesis), surgical removal, or radioactive 131I-iodine to destroy the gland.

[Adrenal glands] Describe the adrenal gland: location, cortex vs medulla (origin and secretion) and blood supply.

Situated on the superior pole of the kidney, embedded in perirenal fat. Outer cortex: derived from mesodermal tissue, secretes steroid hormones. Inner medulla: derived from neuroectodermal tissue, secretes the catecholamines adrenaline and noradrenaline. Highly vascular: the arteriolar network gives rise to a small number of medullary arteries that pass directly through the cortex to the medulla, and many tiny thin-walled sinusoids into which cortical cells secrete steroid hormones. So cortical secretions bathe the medulla cells before leaving the gland via the venous system.

[Adrenal glands] How does the adrenal medulla work (innervation, receptors, adrenaline vs noradrenaline, PNMT)?

The medulla is a modified ganglion of the sympathetic nervous system. Chromaffin cells secrete adrenaline (80%) and noradrenaline (20%) into the blood; they are innervated by splanchnic nerve fibres with acetylcholine (ACh), which acts on nicotinic receptors to increase the rate of catecholamine synthesis and release. The enzyme that converts noradrenaline to adrenaline, phenylethanolamine-N-methyltransferase (PNMT), is maintained by glucocorticoids, so chromaffin cells bathed in steroid-rich blood secrete adrenaline as the primary product.

[Adrenal glands] What are the steps and enzymes in catecholamine synthesis (Fig 48.2)?

Tyrosine -> (tyrosine hydroxylase) dihydroxyphenylalanine (DOPA) -> (amino acid decarboxylase) dopamine -> (dopamine beta-hydroxylase) noradrenaline -> (phenylethanolamine-N-methyltransferase, PNMT) adrenaline.

[Adrenal glands] What are the three zones of the adrenal cortex and what does each secrete?

Zona glomerulosa (outer, thin layer under the capsule): aldosterone (mineralocorticoid). Zona fasciculata (middle, widest band): glucocorticoids such as cortisol and small amounts of adrenal androgens. Zona reticularis (inner): adrenal androgens dehydroepiandrosterone (DHEA) and androstenedione (weak androgens). Each zone expresses a distinct set of enzymes (zone-specific steroidogenesis).

[Adrenal glands] Describe steroid hormone synthesis in the adrenal cortex (source of cholesterol, rate-limiting step, first steroid, StAR, storage).

Many enzymes belong to the cytochrome P450 (CYP) mono-oxidase family. Steroids are made from cholesterol, mainly from plasma LDL, with smaller amounts from de novo synthesis from cellular acetate. The rate-limiting step is the transfer of cholesterol from the outer to the inner mitochondrial membrane, where cholesterol side-chain cleavage enzyme (desmolase, P450scc, CYP11A1) converts cholesterol to the first steroid, pregnenolone. Steroidogenic acute regulatory (StAR) protein is a key cholesterol-shuttling protein. Pregnenolone exits the mitochondria into the microsomal compartment to be converted to other steroids, each shuttling between the two compartments along its pathway. Steroid hormones are lipophilic and not stored; circulating levels are controlled primarily by the rate of synthesis.

[Adrenal glands] Which enzymes are in the steroidogenesis pathway and where are they located (Fig 48.3 key)?

Cholesterol side-chain cleavage enzyme (desmolase, P450scc, CYP11A1): inner mitochondrial membrane (IMM). 3beta-hydroxysteroid dehydrogenase (3beta-HSD): smooth endoplasmic reticulum (SER). 21beta-hydroxylase (CYP21): SER. 11beta-hydroxylase (CYP11B1): IMM. Aldosterone synthase (CYP11B2): IMM. 17alpha-hydroxylase (CYP17): SER. 17,20-lyase (CYP17): SER.

[Adrenal glands] Which steroids belong to the mineralocorticoid, glucocorticoid and sex hormone pathways (Fig 48.3)?

Mineralocorticoid pathway: pregnenolone, progesterone, 11-deoxycorticosterone, corticosterone, aldosterone. Glucocorticoid pathway: 17-alpha-hydroxyprogesterone, 11-deoxycortisol, cortisol. Sex hormone pathway: 17-alpha-hydroxypregnenolone, DHEA, androstenedione (both weak androgens), then testosterone, oestrone, oestradiol, oestriol. Aldosterone is made in the zona glomerulosa, cortisol in the zona fasciculata, DHEA/androstenedione in the zona reticularis.

[Adrenal glands] What happens when an enzyme in steroid biosynthesis is deficient, and what is congenital adrenal hyperplasia (CAH)?

Deficiency of the products downstream of the enzyme, with accumulation of precursors upstream. CAH is a deficiency of an enzyme in steroid biosynthesis; the most common form (>90%) is lack of the CYP21 gene, which encodes 21-hydroxylase. Result: lack of glucocorticoids and mineralocorticoids with excess androgens (elevated ACTH causes hypertrophy of the zona fasciculata and reticularis).

[Adrenal glands] What are the actions of glucocorticoids (cortisol)?

Cortisol is the primary glucocorticoid in humans and also has mineralocorticoid activity. Its metabolic effects generally oppose those of insulin: catabolic effects in muscle and adipose, while in the liver it stimulates glycogen synthesis. It raises plasma glucose primarily by stimulating hepatic gluconeogenesis and, to a lesser extent, by decreasing glucose uptake in skeletal muscle and adipose. At high levels it is anti-inflammatory/immunosuppressive: synthetic steroids (such as prednisolone) with low mineralocorticoid potency are used to control inflammatory or immune conditions. Cortisol is permissive on the actions of catecholamines on carbohydrate metabolism and on cardiovascular regulation.

[Adrenal glands] How is cortisol release controlled (HPA axis, diurnal rhythm, stress)?

By the hypothalamic-pituitary-adrenal axis: hypothalamus (CRH, AVP) -> anterior pituitary (ACTH) -> adrenal cortex (cortisol), with negative feedback. Two factors change the set-point by acting on the hypothalamus: diurnal rhythm (the suprachiasmatic nucleus controls endogenous circadian rhythms) and stress. The diurnal pattern of ACTH release dictates the rhythm of cortisol: lowest at midnight, peak between 6 and 8 am, falling through the rest of the day.

[Adrenal glands] What is the stress response and what are the effects of chronic activation?

In response to emotional stressors (anxiety, fear) or physiological stressors (pain, fever, hypoglycaemia, injury, fluid deprivation, surgery), cortisol and sympathetic nervous activity act together, mobilising biochemical resources (energy molecules) and preparing the body for physical and behavioural readiness. Chronic activation of the stress response can lead to harmful states: muscle wasting, immune suppression, gastric ulcers and hyperglycaemia.

[Adrenal glands] What is the trophic effect of ACTH on the adrenal cortex?

Elevated ACTH levels can lead to hypertrophy of the zona fasciculata and reticularis (e.g. CAH). Low ACTH levels lead to atrophy of the adrenal cortex (adrenal insufficiency; adrenal suppression with chronic use of high-dose steroid medication).

[Adrenal glands] What is aldosterone, what controls its production and what does it do?

Produced exclusively by adrenal glomerulosa cells because they express aldosterone synthase. Production is controlled primarily by angiotensin II (renin-angiotensin system) and plasma K+ levels, with minor influence by ACTH. It regulates renal Na+ (and hence water) reabsorption with K+ excretion, acting on the late distal tubule and collecting duct. Dysregulation may lead to systemic hypertension and hypokalaemia (Conn's syndrome).

[Adrenal glands] Describe the adrenal androgens: secretion pattern, role in women and men, excess and regulation.

DHEA and its sulphated form, with a little androstenedione, are weak androgens. Secretion begins at age 5-6 at a very low level, surges at puberty, peaks at ages 25-30 and then slowly tapers off. In women they contribute 50% of circulating active androgens (pubic and axillary hair growth, sex drive). Androgen excess (ACTH-dependent Cushing's syndrome, CAH) can masculinise females (in utero: ambiguous external genitalia; adult: excessive facial/body hair [hirsutism], acne). In men the contribution is negligible against testicular testosterone. Production is regulated by ACTH, but the androgens do not exert feedback control on the axis.

[Adrenal disorders] Cushing's syndrome (excess glucocorticoids): causes and signs/symptoms (Table 48.1)?

Causes: exogenous glucocorticoid administration; Cushing's disease (hypersecretion of anterior pituitary ACTH); adrenal adenoma; ectopic ACTH production (e.g. small cell carcinoma). Signs and symptoms: muscle wasting (limbs), truncal obesity, thin skin, poor wound healing, easy bruising, acne, female hirsutism (ACTH-dependent), glucose intolerance, hypertension.

[Adrenal disorders] Conn's syndrome (excess mineralocorticoids): cause and signs/symptoms (Table 48.1)?

Cause: aldosterone-secreting adrenal adenoma. Signs and symptoms: hypertension and hypokalaemia.

[Adrenal disorders] Adrenal insufficiency: causes of Addison's disease (primary) and secondary adrenal insufficiency, and signs/symptoms (Table 48.1)?

Addison's disease (primary): destruction of adrenal tissue, often autoimmune. Secondary adrenal insufficiency: hyposecretion of anterior pituitary ACTH. Signs and symptoms: weakness, fatigue, lethargy, dehydration, hypotension, hyponatraemia, hyperkalaemia.

[Pathogens: basics] Define free-living microbe, parasite, commensal (normal flora), pathogen, primary pathogen, opportunistic pathogen, infection and pathogenesis.

Most microbes are free-living and obtain energy from light and the oxidation of inorganic or dead organic matter. A parasite lives in or on a living host from which it obtains nourishment. Most parasitic microbes are commensal or normal flora and are generally harmless or beneficial. A pathogen is any microbial agent that can cause disease in a host. Primary pathogens invade tissues, overcome host defences and cause disease. Opportunistic pathogens are commensal flora that cause disease when host defences are impaired or compromised (e.g. breaking the skin). Infection = the process of invasion of the body; pathogenesis = the mechanism of causing infection.

[Pathogens: basics] What are the five steps to establishing a successful infection?

1 Attachment and entry: pathogens attach to the host and gain entry. 2 Dissemination and multiplication: they replicate and move away from the initial site of colonisation. 3 Evasion: hosts have protective mechanisms and pathogens have evolved ways to circumvent them. 4 Exit: pathogens leave the host to spread to fresh hosts, often in normal secretions (saliva, mucus, faeces). 5 Damage: either directly from the pathogen or as collateral damage from the host response.

[Pathogens: basics] What are Koch's postulates (1890) and what is their limitation?

1 The organism must be present in every case of the disease. 2 It must be isolated from the diseased host and grown in pure culture. 3 The pure culture must induce the disease in an experimentally infected host. 4 The organism must be recoverable from the experimentally infected host. Well established for living bacterial pathogens, but their applicability to pathogens such as viruses and prions is problematic.

[Bacteria] What are the defining features of bacteria (Fig 58.1) and how do they colonise the host?

Bacteria are prokaryotes with virulence determinants for pathogenesis. All have a peptidoglycan layer around an inner membrane; Gram-negative bacteria also have an external (outer) membrane; some have a tough polysaccharide capsule against environmental stresses. Pili and flagella are used for attachment and motility. Multiple secretion systems release exotoxins, enzymes and other virulence factors. LTA (Gram-positive) and LPS (Gram-negative) are potent activators of innate immune responses. Colonisation of host tissue is the initial step of infection, via interaction between specific host cell receptors and bacterial surface adhesins. Examples: pili (fimbriae) and flagella give motility and adhesion in Vibrio cholerae and Campylobacter jejuni; Streptococcus mutans makes an exopolysaccharide that lets it adhere to teeth.

[Bacteria] How do bacteria avoid host defences (example: Neisseria meningitidis)?

N. meningitidis produces a thick extracellular polysaccharide capsule to avoid phagocytosis by macrophages. It also avoids antibody-mediated immune responses by altering its surface antigens, which allows longer survival in the host and makes vaccine development difficult.

[Bacteria] How do bacteria cause tissue damage (endotoxin vs exotoxins)?

Endotoxin = lipopolysaccharide (LPS), a component of the outer membrane of Gram-negative bacteria, and lipoteichoic acid (LTA), a component of the cell wall of Gram-positive bacteria; both are potent activators of immune and inflammatory responses. Exotoxins are secreted by pathogenic bacteria and have diverse effects: Shiga toxin (Shigella) inhibits host cell protein synthesis; Staphylococcal haemolysin forms lytic pores in blood cells.

[Viruses] What are viruses and what is their basic structure (capsid, nucleocapsid, envelope)?

Viruses are obligate intracellular parasites: they cannot make energy or proteins independently. They are latent until they interact with the appropriate receptor on a host cell and are incorporated; the cell's biochemical processes are then co-opted to make more viral particles (virions). At the most basic the virion is the genome (single- or double-stranded DNA or RNA) surrounded by structural proteins, enzymes and nuclear binding proteins (the capsid) = the nucleocapsid, released when the cell lyses. The capsid protects the genetic material, so these viruses can survive long in the environment and pass through the GI tract. Some viruses escape by budding from the cell membrane: the particle is enclosed in a protein matrix and a lipid bilayer studded with glycoproteins (the envelope).

[Viruses] What do envelope glycoproteins do, and how are enveloped viruses spread?

Envelope glycoproteins aid attachment to the host cell (e.g. GP120 and GP41 in HIV) or act as virulence factors (e.g. haemagglutinin and neuraminidase in influenza). Enveloped virions are very sensitive to inactivation, so they are generally passed from person to person by the exchange of fluids.

[Viruses] What are viral virulence mechanisms and how do viruses damage the host?

Virulence mechanisms (mainly to evade host detection and eradication): sequestration, blockade of antigen presentation, cytokine evasion, inhibition of apoptosis, antibody evasion. Damage: disruption of host cell processes, lysis of host cells, or damage from a persistent and ongoing engagement of host inflammatory or immune responses. Example: HIV infects immune cells, which are lysed or destroyed, causing immunosuppression and acquired immune deficiency syndrome (AIDS).

[Fungi] What are fungi, how do they cause disease (mycoses) and what are examples?

Fungi are eukaryotic organisms (yeasts and moulds); only a small percentage are routinely associated with human disease. Most fungal pathogens are opportunistic and acquired from the environment (main exceptions: dermatophytes and Candida albicans). Candida reproduces by budding, has fimbriae (pili) and adhesins for attachment, and secretory vesicles that secrete virulence factors such as proteases and phospholipases. Mycoses (fungal infections) are separated by the tissue depth of initial colonisation. Systemic mycoses can affect many tissues and are often internal, inhaled into the lungs or spreading through mucosal sites such as the female genital tract. Usually some depletion of host defences is needed (disrupted normal flora, immunosuppression), e.g. pneumonia from Aspergillus in chemotherapy patients, Pneumocystis jiroveci pneumonia in AIDS.

[Fungi] How do pathogenic fungi avoid clearance and damage the host?

Avoid clearance: by killing the engulfing immune cell (Candida albicans produces long hyphae that skewer the engulfing macrophage) or by avoiding phagocytosis (Cryptococcus neoformans produces a polysaccharide capsule). Damage: breakdown of host tissue by secreted proteases and lipases, and secretion of metabolites such as alkaloids, which can cause acute organ damage.

[Parasites] What are protozoa and helminths (definition, size, groups, examples)?

Medical parasitology deals with invertebrate animals capable of causing disease. Protozoa: simple unicellular microorganisms, 2-100 μm; include amoebae, flagellates and ciliates; agents of malaria (Plasmodium), toxoplasmosis (Toxoplasma gondii) and leishmaniasis (Leishmania). Helminths: multicellular macroscopic parasites (metazoa, which include helminths and arthropods); nematodes are cylindrical-bodied worms (hook worm, pin worm); platyhelminths are flat-bodied worms (flukes, tapeworms).

[Parasites] How do protozoa and helminths enter and attach to the host?

Infections are virtually always acquired from an exogenous source. Entry: oral ingestion (e.g. Giardia lamblia, causes diarrhoea) or penetration through the skin, often via an arthropod vector (malaria is transmitted by female mosquitoes). Attachment is varied: adhesins and receptor-mediated attachment like bacteria and viruses (e.g. Leishmania mexicana attaches via CR2 on host cells) or mechanical means (e.g. G. lamblia uses a ventral disk that clamps onto the host intestinal epithelium).

[Parasites] How do parasites cause disease and avoid immune responses?

Many protozoa secrete molecules such as phospholipases and proteases that directly damage host cell membranes. Pathology from helminth infections is generally due to the size and longevity of the parasite, which can disrupt host physiological functions (e.g. blocking bile ducts, compressing parts of the central nervous system). Evasion: many parasites use mechanisms common to bacteria; Plasmodium can vary surface antigens, mimic host antigens, directly suppress immune responses and reside intracellularly.

[Prions] What are prions and how do they cause disease?

Transmissible aberrant proteins thought to cause spongiform encephalopathies in mammals, notably Creutzfeldt-Jakob disease (CJD) and Kuru in humans. The normal cellular prion protein (PrPc) converts when it contacts the misfolded isoform (PrPsc) and becomes resistant to degradation by proteases, so the aberrant protein builds up and causes disease. Prions can be isolated from other tissues but only the brain shows pathology. Spread is thought to be by ingestion or injection of brain tissue from contaminated individuals; a genetic mutation causes familial prion disease. Because the prion protein is a host protein, no immune response is initiated: lack of an inflammatory response is characteristic.

[Antimicrobial drugs] What is the principle behind antibacterial and antiviral drugs?

Antibacterials exploit differences between eukaryotic host cells and prokaryotic bacteria: penicillins target formation of the bacterial cell wall (inhibiting the transpeptidases that cross-link peptidoglycans) and macrolides target bacterial ribosomes. Viral infections are harder to manage by targeting biochemical differences; targets include viral DNA polymerases, neuraminidases and HIV reverse transcriptases.

[Antimicrobial drugs] Cell wall antibacterials: mode of action and uses of penicillins and cephalosporins (Table 58.1)?

Penicillins (amoxicillin, phenoxymethyl penicillin, flucloxacillin): inhibit cross-linking of peptide side chains. Uses: phenoxymethyl penicillin - tonsillitis; flucloxacillin - impetigo, cellulitis; amoxicillin - chest infections, otitis media, UTIs. Cephalosporins (cefalexin, cefotaxime, cefaclor): bind beta-lactam-binding sites and inhibit cell wall synthesis. Uses: septicaemia, pneumonia, meningitis, biliary tract infections, peritonitis, UTIs.

[Antimicrobial drugs] Protein synthesis-inhibiting antibacterials: tetracyclines, macrolides, aminoglycosides - mode of action and uses (Table 58.1)?

Tetracyclines (tetracycline, doxycycline): inhibit protein synthesis by interfering with tRNA binding. Uses: Chlamydia, exacerbation of chronic bronchitis (Haemophilus influenzae), periodontal disease, acne, respiratory and genital Mycoplasma infections; doxycycline in malaria prophylaxis. Macrolides (erythromycin, clarithromycin): prevent translocation of the bacterial ribosome along the mRNA, preventing protein synthesis. Uses: alternative in penicillin-allergic patients, respiratory infections, whooping cough, Legionnaires' disease, Campylobacter enteritis. Aminoglycosides (gentamicin): irreversibly bind bacterial ribosomes, inhibiting protein synthesis. Uses: gentamicin in septicaemia, meningitis, acute pyelonephritis, endocarditis.

[Antimicrobial drugs] Sulphonamides/trimethoprim and quinolones: mode of action and uses (Table 58.1)?

Sulphonamides and trimethoprim: inhibit folate synthesis and reduce the precursors of DNA and RNA. Uses: pneumonia in AIDS patients, toxoplasmosis and nocardiosis, acute exacerbation of chronic bronchitis, otitis media, UTIs. Quinolones (ciprofloxacin, ofloxacin): inhibit bacterial DNA gyrase. Uses: Pseudomonas aeruginosa, Haemophilus influenzae, Campylobacter.

[Antimicrobial drugs] Antiviral and antifungal agents: mode of action and uses (Table 58.2)?

Antivirals: aciclovir inhibits herpes virus DNA polymerase (uses: herpes simplex and herpes varicella); famciclovir is also antiviral; zanamivir is a neuraminidase inhibitor that prevents entry and release of viral particles from host cells (use: influenza). Imidazoles (clotrimazole) and triazoles (itraconazole, fluconazole): inhibit P450-dependent demethylase, which converts lanosterol to ergosterol; accumulation of lanosterol disrupts the fungal membrane (use: fungal infections). Terbinafine inhibits conversion of squalene to lanosterol; accumulation of squalene causes cell death. Griseofulvin interferes with fungal microtubules and nucleic acid synthesis; suitable for tinea infections but not candidiasis.

[Innate and adaptive immunity] What are the major functions of the immune system and the main features of innate vs adaptive immunity?

Major functions: recognition of pathogens (by interacting with microbes and their components) and defence of the body by eliminating them. Innate immunity: quickly activated when infection occurs, remains the same on repeated exposure to the same microbe, moderately efficient. Adaptive (acquired) immunity: more slowly activated (information about the pathogen must be taken to naive T and B lymphocytes in local secondary lymphoid tissues, e.g. lymph nodes), efficacy improves on repeated exposure, highly efficient once activated and gives specific responses tailored to each type of microbe.

[Innate recognition] What triggers innate recognition?

Innate immunity gives a general response to categories of microbe. It is triggered by recognition of complement proteins that have opsonised the pathogen, and of chemical structures characteristic of microbes (pathogen-associated molecular patterns, PAMPs). Further recognition is achieved by receptors on natural killer (NK) cells.

[Innate recognition] What is complement and how does complement opsonisation work?

Complement is a collection of about 30 proteins in the circulation and tissue fluids, initially described by their ability to 'complement' the effects of antibodies. Various complement proteins act as activation enzymes, defence molecules and regulatory proteins. Activation is triggered by infection and immune activation and occurs as a cascade (chain reaction) with built-in amplification. Fragments of complement bind to the pathogen surface, 'opsonising' it so it can be seen by immune cells with complement receptors (such as macrophages).

[Innate recognition] What are PAMPs and PRRs, and what do cell-associated and soluble PRRs do?

PAMPs are chemical structures characteristic of microbes (e.g. bacterial lipopolysaccharide and lipoproteins, viral double-stranded RNA/DNA, fungal mannoprotein, Toxoplasma gondii for parasites). They interact with pattern recognition receptors (PRRs): toll-like receptors (TLRs), C-type lectin receptors (CTLRs), NOD-like receptors (NLRs), RIG-like helicase receptors (RLRs) and scavenger receptors, or soluble PRRs. Cell-associated PRRs enable phagocytosis of PAMPs and associated microbes, activation of cells and release of inflammatory mediators that amplify the immune response. Soluble PRRs can directly attack the microbe, enhance phagocytosis of PRR-bound PAMPs and activate the proteolytic cascade, resulting in lysis of the microbe.

[Innate recognition] How do NK cells recognise and kill target cells (receptor classes, HLA class I, MIC-A/B)?

NK cells recognise targets via a large repertoire of activating and inhibitory receptors in an antigen non-specific manner, so they act quickly. Three main receptor classes: killer cell immunoglobulin-like receptors (KIRs) and killer cell lectin-like receptors (KLRs), largely inhibitory, which detect levels of HLA class I; and natural cytotoxicity receptors (NCRs), exclusively activating. Healthy cells constantly express HLA class I, which binds inhibitory receptors and blocks stimulatory signalling. If HLA class I expression is reduced (e.g. a virus avoiding detection), inhibitory signalling is absent and the cell is killed. If a stressed cell up-regulates HLA class IB-like molecules MIC-A and MIC-B, activating signals (e.g. via the NKG2D receptor) override inhibitory signalling and the target is killed.

[Adaptive recognition] What is adaptive recognition and which cells carry it out?

Adaptive immunity recognises antigens specific to each type of microbe using antigen receptors that are clonally expressed by lymphocytes. Lymphocytes are therefore the cell type responsible for adaptive immune recognition.

[Adaptive recognition] Describe the B cell receptor (BCR): structure, specificity, signalling and co-receptors.

The BCR antigen receptor is surface immunoglobulin (sIg: IgM, G, A, D, E) that binds native antigen. All sIg on one B cell have identical antigen-combining sites (one B cell = one antigen epitope). When activated, the B cell differentiates into a plasma cell that secretes antibodies (soluble immunoglobulins) with the same specificity as the sIg. sIg has a tail of only three amino acids and cannot signal into the cell, so the BCR also contains Ig-beta (CD79b) and Ig-alpha (CD79a), which enable signalling (signal 1). For full activation a second signal is required (probably a safeguard): the BCR co-receptor complex (CD19, CD21 [CR2], CD81, CD225) can engage molecules such as complement on the same target surface (e.g. a bacterium).

[Adaptive recognition] What are clonal selection and memory cells in B cell responses?

The body has millions of B cell clones, each with Ig with a different antigen-combining site. An antigen interacts with the clones whose sIg bind it with the highest affinity (clonal selection). Activation causes proliferation of those clones, and some cells are kept as memory cells. On re-exposure to the same antigen, memory cells are reactivated more rapidly and in larger numbers than in the primary response, giving the faster and bigger response that characterises adaptive immunity.

[Adaptive recognition] Describe the T cell receptor (TCR): chains, structure, co-receptors and how it differs from the BCR.

Composed of two polypeptide chains (alpha and beta, or occasionally gamma and delta), both anchored in the surface membrane; there is no secreted form (T cells make no equivalent of antibodies). Each chain is about the size of an immunoglobulin light chain and has two immunoglobulin-like domains, so the TCR is a member of the immunoglobulin superfamily. The membrane-proximal domain is constant (C-alpha, C-beta); the distal domains are variable (V-alpha, V-beta) and form a single antigen-combining site (the TCR has one, sIg on B cells has two). Co-receptors: CD3 (delta, epsilon, gamma, zeta); CD4 on Th cells binds MHC II; CD8 on Tc cells binds MHC I.

[Adaptive recognition] What do T helper and T cytotoxic cells do, and what do TCRs recognise (APC, HLA/MHC)?

Th cells help other immune cells: activate B cells to produce antibodies and stimulate the phagocytic and killing activity of macrophages. Tc cells kill tissue cells infected by microbes. The TCR cannot bind microbes or their antigens directly; it binds only fragments of antigenic proteins (antigen peptides) associated with tissue cells. Cells presenting antigen peptides to T cells are antigen presenting cells (APC): in the APC cytoplasm microbial proteins are broken down to peptides, some of which associate with specialised peptide-binding proteins that hold them on the APC surface. In humans these are HLA proteins (human leucocyte antigens); more generally MHC proteins (major histocompatibility complex).

[Defence: extracellular microbes] How does the immune system defend against extracellular microbes such as bacteria (innate and antibody-mediated)?

Innate: leucocytes (e.g. neutrophils) and epithelial cells produce antimicrobial peptides called defensins that directly damage certain microbes. Adaptive: B cells secrete antibodies (mainly IgG) that can defend simply by binding antigens, e.g. neutralising bacterial toxins or viruses. Antibodies also activate other components via their Fc regions: IgM and IgG activate the complement system and virtually all immune cells express Fc receptors (FcR). Defence mechanisms: neutralisation, opsonisation, lysis, phagocytosis, intracellular digestion.

[Defence: extracellular microbes] How do macrophages and neutrophils recognise, engulf and destroy microbes, and how do T helper cells support them?

Macrophages are tissue-resident cells derived from blood monocytes; neutrophils circulate in the blood and are recruited to sites of infection and inflammation. Acute phase proteins C-reactive protein and mannose-binding lectin opsonise bacteria for uptake and activate complement. Phagocytes recognise microbes via PRRs, complement receptors (CR) or antibody receptors (FcR), triggering phagocytosis: the microbe is internalised into a membrane-bound vesicle (phagosome). Lysosomes with digestive enzymes fuse with the phagosome and degrade the microbe; the phagocyte also produces reactive oxygen species that enter the phagosome, help degradation and produce a pH suitable for the enzymes. CD4 T helper cells coordinate the response: Th17 cells (secrete IL-17) support neutrophil function; Th1 cells (secrete interferon-gamma) support macrophage function.

[Defence: intracellular microbes] How does the immune system defend against intracellular microbes such as viruses (general strategy)?

Viruses lack the metabolic machinery to replicate themselves and must infect cells. Some defences target free virus particles (destroy them or neutralise their ability to infect cells); others target infected cells (block virus replication or kill the infected cell). The early innate response by interferons and natural killer (NK) cells limits the growth and spread of infection. The adaptive response by CD8 cytotoxic T cells takes longer to activate but is highly efficient and may clear the infection.

[Defence: intracellular microbes] What do type 1 interferons do?

Virus-infected cells produce and secrete type 1 interferons (IFN-alpha and IFN-beta). They bind receptors on neighbouring cells and trigger an antiviral state (resistance to virus replication) by inducing enzymes that degrade viral mRNA and inhibit protein synthesis. They also enhance expression of HLA class I (making cells better targets for CD8 cytotoxic T cells) and activate NK cells.

[Defence: intracellular microbes] How do NK cells recognise virus-infected target cells (two mechanisms, ADCC)?

(1) Through their large repertoire of activating and inhibitory receptors (Chapter 59). (2) Through IgG antibodies that bind native viral antigens on the infected cell surface: NK cells have Fc receptors for IgG that bind the antibodies, attaching the NK cell to the infected cell, which it then kills. This is antibody-dependent cellular cytotoxicity (ADCC).

[Defence: intracellular microbes] How do NK cells and Tc cells kill their targets (perforin, granzymes, Fas)?

NK cells and Tc cells identify targets differently (Tc cells recognise HLA class I-associated peptides) but kill by similar mechanisms, both inducing target cell apoptosis. They have granules with two proteins released onto the target surface: perforins (similar to C9 of the complement membrane attack complex) are inserted into the target membrane and help uptake of granzymes; granzymes enter the cytoplasm and activate caspase enzymes that start the apoptosis pathways. Tc cells also express Fas-ligand, which interacts with Fas on the target cell surface and also activates the apoptosis pathways. (Th1 cells support this response via IL-2 and IFN-gamma.)

[Defence: extracellular parasites] How is the body defended against extracellular parasites such as helminths?

Parasites are too large to be engulfed by phagocytes or killed by CD8 cytotoxic T cells and NK cells. They are removed by an inflammatory response from eosinophils, mast cells and basophils, mediated by IgE (an immunoglobulin isotype produced by B cells; Th2 cells and IL-4/IL-5 are involved). Mechanisms: inflammation, expulsion, extracellular digestion.

[Defence: extracellular parasites] How do eosinophils attack parasites?

Eosinophils mediate extracellular digestion. They express FcR that bind IgG or IgE and also complement receptors, so opsonisation of a worm by antibodies and C3b lets many eosinophils bind to its surface. Their cytoplasmic granules contain digestive proteins (e.g. major basic protein, eosinophil cationic protein), which are released onto the parasite surface.

[Defence: extracellular parasites] How do mast cells become activated and what do they release?

Mast cells are tissue cells with high-affinity FcR that bind IgE from the surrounding tissue fluid, so they become coated with IgE and acquire its antigenic specificities. When an antigen binds two or more IgE antibodies on the surface at the same time (cross-links/bridges the IgE), inflammatory mediators are released. Release can also be triggered by complement peptides C3a and C5a (which also attract and activate neutrophils). Preformed mediators stored in granules (histamine, heparin, tryptase) are released immediately. Mast cell activation also induces synthesis of new mediators released over a longer time: leukotrienes and prostaglandins (from arachidonic acid) and cytokines.

[Defence: mucosal microbes] How are mucosal microbes neutralised (IgA, poly-Ig receptor, secretory piece, milk)?

By IgA, an immunoglobulin isotype produced by B cells. IgA is present in blood and internal tissue fluids as a monomer. Dimeric IgA is transported across mucosal epithelium into secretions of the gastrointestinal, respiratory and genitourinary tracts via the poly-Ig receptor on mucosal epithelial cells; part of the receptor stays attached to the secreted IgA dimer and is called the secretory piece. IgA is also secreted into milk by the mammary gland, so the mother's IgA protects the gut of suckling infants. B cells make IgA under stimulation by IL-5 (from CD4 Th2 cells) or TGF-beta (from CD4 T regulatory cells).

[Integration: innate response] What is the immediate local innate response to infection?

The first infection by a microbe gives a primary immune response; normally pathogens must penetrate surface epithelial barriers (e.g. the epidermis). Immune components resident in the tissues (macrophages, mast cells, complement proteins) give an immediate local innate response and generate inflammatory mediators (augmented by mast cell activation) that attract more leucocytes and serum proteins from the blood. Macrophages phagocytose bacteria using PRRs and after opsonisation with the acute phase proteins C-reactive protein and mannose-binding lectin, which also activate complement to generate C3a and C5a. Mast cells release preformed mediators and, over a longer period, newly formed mediators (leukotrienes, prostaglandins, cytokines); this release can be triggered by C3a and C5a, which also attract and activate neutrophils.