The Immune System: Innate and Adaptive Body Defences

A comprehensive vocabulary review deck covering the cellular components, chemical barriers, inflammatory mediators, complement system, and adaptive immune mechanisms (both cell-mediated and humoral) from the lecture on the immune system.

Human Leucocyte Antigen (HLA) system

A unique group of membrane glycoproteins belonging to the Major Histocompatibility Complex that allows immune cells to recognize cells as self and distinguish them from foreign microorganisms, abnormal cells, or toxins.

Antigen

A foreign substance that enters the body and triggers an immune response.

Antibody

A Y-shaped protein molecule secreted by plasma cells that binds specifically to target antigens to facilitate their removal or destruction.

Acid mantle

An innate chemical surface barrier found on the skin, stomach lining, and vaginal walls that inhibits bacterial multiplication and growth.

Mucin

A protein dissolved in water that forms the thick, protective mucus lining the respiratory and digestive tracts to trap invading microorganisms for removal.

Defensins

Antimicrobial peptides secreted in skin and mucous membranes capable of killing a wide range of microorganisms in exposed epithelial areas.

Dermacidin

An antimicrobial peptide excreted in eccrine sweat that is directly toxic to bacteria residing on the skin surface.

Phagocytes

Specialized white blood cells developed from hematopoietic stem cells in the red bone marrow that are capable of engulfing and absorbing dead cells, debris, and pathogens.

Neutrophils

The most abundant white blood cells ($50\text{--}70\%$ of circulating leucocytes) containing defensins and lysosomes that destroy bacteria and fungal pathogens during acute inflammation.

Monocytes

Agranular leucocytes comprising $3\text{--}8\%$ of circulating white blood cells that leave the bloodstream and migrate into tissues to differentiate into macrophages.

Macrophages

Tissue-resident phagocytic cells derived from monocytes that engulf cellular debris, intracellular bacteria, and viruses, and clean up damaged tissue sites so healing can begin.

Dendritic cells

Sentinel phagocytic cells located throughout tissues that engulf pathogens and cellular debris while serving as vital messengers to activate T cells in adaptive immunity.

Phagosome

A cytoplasmic vesicle formed inside a phagocyte when pseudopods surround and engulf an extracellular pathogen or cellular debris.

Phagolysosome

A hybrid vesicle formed by the fusion of a lysosome with a phagosome, within which acid hydrolase enzymes and toxic compounds digest engulfed particles.

Natural killer (NK) cells

Non-specific granular lymphocytes that destroy abnormal cells lacking normal Major Histocompatibility Complex proteins (such as virus-infected or cancer cells) via apoptosis or cytokine release.

Apoptosis

A programmed, organized series of cellular steps resulting in cell death without inducing uncontrolled lysis or surrounding tissue damage.

Mast cells

Connective tissue immune cells bearing IgE and IgG receptors that release mediator granules, predominantly histamine and heparin, to initiate local acute inflammation.

Histamine

An inflammatory mediator released by mast cells and basophils that promotes immediate arteriolar vasodilation and increases capillary permeability to form exudate.

Heparin

A chemical mediator released by mast cells that allows plasma and immune cells to leak out of blood vessels into surrounding connective tissues.

Basophils

Circulating granulocytes comprising less than $1\%$ of leucocytes that recognize PAMPs, release histamine, and secrete interleukin-4 ($\text{IL-4}$) and interleukin-13 ($\text{IL-13}$) to combat parasites and regulate allergy.

Eosinophils

Granulocytes found in loose connective tissues that release enzymes and free radicals directly onto the surfaces of parasitic worms, as well as proteins that neutralize heparin.

Leukotrienes

Sulphur-containing lipid mediators produced via lipoxygenase that cause smooth muscle contraction and prolonged increases in vascular permeability during later stages of inflammation.

Prostaglandins (PG)

Lipid-derived inflammatory mediators associated with cyclooxygenase-2 ($\text{COX-2}$) that induce increased vascular permeability, smooth muscle contraction, neutrophil chemotaxis, and pain.

Platelet-activating factor (PAF)

A mediator formed by cleaving fatty acids from membrane phospholipids that induces endothelial retraction, vascular permeability, leucocyte adhesion, and platelet activation.

Bradykinin

An inflammatory peptide that stimulates unencapsulated sensory nerve endings, serving as the primary noxious stimulus responsible for pain during inflammation.

Leukocytosis

The rapid entry of neutrophils into the bloodstream from red bone marrow in response to leukocytosis-inducing factors released by injured tissue cells.

Margination

The phenomenon wherein circulating neutrophils adhere and cling to the inner endothelial lining of capillaries near an injured or inflamed site.

Diapedesis

The process by which chemical mediators cause neutrophils and other leukocytes to flatten and squeeze between endothelial cells out of capillary walls into tissues.

Chemotaxis

The directed migration of white blood cells along a chemical concentration gradient toward the highest concentration of inflammatory chemicals at the site of injury.

Exudate

Protein-rich fluid and clotting factors that leak from capillaries into interstitial tissue spaces due to increased vascular permeability, causing swelling (edema).

Interferons

Antimicrobial proteins produced and secreted by uninfected or virus-infected cells that protect neighboring uninfected cells from viral entry and regulate antiviral defenses.

Classical pathway

A complement activation pathway initiated when circulating complement proteins bind directly to antigen-antibody complexes on the pathogen surface.

Lectin pathway

A complement activation pathway in which lectins bind specific sugars on the surface of pathogens, allowing the innate immune system to recognize and attack them.

Alternative pathway

A complement activation pathway that occurs spontaneously when complement proteins attach directly to surface molecules on a pathogen or damaged cell without antibodies.

Pyrogens

Chemical substances released by leucocytes and macrophages exposed to foreign pathogens that reset the hypothalamic thermostat to elevate body temperature above $37\,^\circ\text{C}$.

Immunological surveillance

The ongoing monitoring and destruction of abnormal or virus-infected cells in peripheral tissues by circulating natural killer (NK) cells.

Antigen presenting cells (APCs)

Cells that engulf pathogens, process their protein fragments into peptides, and display them bound to surface MHC molecules to T lymphocytes; includes dendritic cells, macrophages, and B cells.

Thymocytes

Immature T cells originating in red bone marrow that migrate to the cortex, corticomedullary junction, and subcapsular space of the thymus gland to mature into functional T lymphocytes.

CD4 T cells

T lymphocytes displaying cell differentiation glycoprotein 4 receptors that differentiate into helper T cell lineages (Th1, Th2, Th17, Treg, and follicular helper T cells) to coordinate immune defenses.

CD8 T cells

T lymphocytes displaying cell differentiation glycoprotein 8 receptors that are activated in secondary lymphoid organs to become cytotoxic ('killer') T cells.

Cytotoxic T cells

Effector CD8 T lymphocytes that directly attack and destroy virus-infected, cancerous, or foreign cells by releasing perforins and granzymes or triggering apoptosis.

Perforins

Lytic proteins released by cytotoxic T cells and natural killer cells that insert into the plasma membrane of a target pathogen or abnormal cell to create pore channels.

Granzymes

Protein-digesting enzymes released by cytotoxic T cells that pass through perforin membrane channels into target cells to degrade proteins and induce apoptosis.

Regulatory T cells (Treg)

A specialized subset of helper T cells that release inhibitory cytokines to dampen, regulate, and terminate immune responses, preventing autoimmune damage.

Follicular helper T cells

Helper T lymphocytes that reside within lymph node follicles to support germinal center maintenance and facilitate B cell differentiation into plasma and memory cells.

Plasma cells (B1 lymphocytes)

Effector B cells that synthesize and secrete large quantities of circulating antibodies against specific pathogens, typically surviving for only $4\text{--}5\text{ days}$.

Memory B cells (B2 lymphocytes)

Long-lived B cells produced during primary immune activation that preserve antigen specificity, mounting an immediate and heightened antibody response upon re-exposure.

Immunoglobulin M (IgM)

A pentameric antibody that functions as the first class secreted by plasma cells during a primary immune response, serves as an active complement activator, and acts as a potent agglutinin.

Immunoglobulin A (IgA)

A dimeric antibody known as secretory IgA present in bodily secretions (saliva, sweat, milk, intestinal juice) that prevents pathogens from adhering to epithelial surfaces.

Immunoglobulin D (IgD)

A monomeric antibody anchored to the external surface of B lymphocytes that serves primarily as an antigen receptor for initial B cell activation.

Immunoglobulin G (IgG)

The most abundant antibody in plasma ($75\text{--}85\%$ of circulating immunoglobulins) that provides the main response in secondary exposures, activates complement, and crosses the placenta.

Immunoglobulin E (IgE)

A monomeric antibody whose stem end binds to mast cells and basophils, triggering degranulation and histamine release during allergic reactions or chronic intestinal parasitic infections.

Rh(D) Immunoglobulin-VF

An anti-D injection containing passive exogenous IgG antibodies administered to an Rh-negative mother after delivering an Rh-positive baby to prevent maternal antibody formation in subsequent pregnancies.

Active naturally acquired immunity

Immunity that develops when an individual is naturally exposed to a live pathogen, experiences clinical or subclinical infection, and develops their own antibodies and memory cells.

Active artificially acquired immunity

Immunity produced intentionally through vaccination with dead, inactivated, or attenuated pathogens, stimulating the body to generate specific antibodies and memory lymphocytes.

Passive naturally acquired immunity

Immunity acquired through the direct transfer of maternal antibodies to a developing fetus via the placenta (IgG) or to an infant through breast milk (secretory IgA).

Passive artificially acquired immunity

Temporary immunity provided by administering preformed exogenous antibodies (gamma globulins) via injection, including antivenom, antiserum, or antitoxin preparations.