Principles of Disease Final Exam Review Flashcards

Foundations of Cellular Injury and Degeneration

Non-lethal or sub-lethal cell injury manifests as cell degeneration, which is characterized by biochemical functional abnormalities, recognizable structural changes, or a combination of both. While degeneration is potentially reversible, it can serve as a precursor to necrosis or cell death. When degeneration leads to abnormal function, it carries clinical significance. In some instances, tissues and organs may reach a static, sub-optimal functional state known as the compensated state. In this condition, inefficient function is maintained at a level sufficient to preserve life, as the body invokes compensatory mechanisms to maintain homeostasis.

Lethal cell injury, or necrosis, is irreversible and marked by recognizable microscopic changes. It may progress from sub-lethal injury or occur immediately if the injury is of sufficient magnitude. Necrotic cells cease all function. Necrosis refers specifically to local cell death within a living organism, which is distinct from somatic death, the death of the entire body. A primary driver of cellular health is the production of ATP via oxidative phosphorylation, a process where ADP is phosphorylated through the oxidation of substances in the respiratory chain. Because this is an oxidative process, oxygen is a strict requirement. Impairment in energy production occurs under conditions of hypoxia, hypoglycemia, enzyme inhibition, and the uncoupling of oxidative phosphorylation.

Hypoxia refers to insufficient oxygen within cells. Oxygen travels through the trachea, bronchi, and bronchioles to the alveoli. Alveoli are surrounded by capillaries where oxygen is absorbed into the blood and bound to hemoglobin. Oxygenated blood travels via pulmonary veins to the left heart and is pumped through systemic circulation. Oxygen is released at the tissue level from capillaries and diffuses into cells. Hypoxemia, a decreased ability of the blood to carry oxygen, can result from anemia (low red blood cells or insufficient hemoglobin) or carbon monoxide (CO) poisoning, where hemoglobin is altered and cannot bind oxygen. Additionally, at high altitudes, the atmospheric oxygen content is decreased, leading to insufficient inspired air. Hypoglycemia, or low blood glucose, causes ATP depletion because glucose is the primary substrate for energy in most cells and the exclusive energy source for brain neurons under normal conditions. ATP depletion also results from enzyme inhibition, such as the toxin cyanide interfering with cytochrome oxidase, or the uncoupling of oxidative phosphorylation, which alters the organization of enzymes on mitochondrial membranes.

Mechanisms of Cell Degeneration and Necrosis

ATP depletion leads to several detrimental effects, starting with intracellular water accumulation. Cells must expend energy to pump sodium and water out to maintain higher internal solute concentrations. Lack of ATP causes the failure of energy-dependent sodium pumps, leading to "cloudy swelling," where the cytoplasm appears granular or vacuolated. This electrolyte imbalance inhibits enzymes and alters electrical activity. Mitochondria also swell, physically uncoupling oxidative phosphorylation and further impairing ATP synthesis by disrupting respiratory enzymes on the cristae. Cells then switch to anaerobic glycolysis, which produces lactic acid, lowers intracellular pH, and damages lysosomal membranes. The subsequent release of lysosomal enzymes into the cytoplasm exacerbates injury. Cells with high metabolic rates and oxygen demands, such as brain cells, are affected first; thus, changes in consciousness are common early signs of hypoxia or hypoglycemia.

Impaired cell membrane function is another critical mechanism. The membrane, composed of phospholipids and proteins, can be damaged by free radicals (oxidative stress), complement system activation, or direct lysis. Direct lysis is induced by enzymes with lipase-like activity, viruses that insert into the membrane or initiate immune responses, and physical or chemical agents like extreme temperatures or solvents. Damage leads to a loss of selective permeability and structural integrity, causing the cell to rupture. Less severe injury may result in shape changes, such as red blood cells becoming spheroidal. Damaged membranes can be deposited in the cytoplasm as lipofuscin, a golden-brown pigment consisting of phospholipid and protein complexes from free-radical peroxidation. Commonly seen in the liver, heart, and neurons, lipofuscin is considered a normal aging change and has no deleterious effect on function.

Intracellular accumulations occur due to inadequate removal, excessive production, or exogenous deposition. Fatty degeneration (steatosis) is the accumulation of triglycerides (TGs) within parenchymal cells, most commonly in the liver. Free fatty acids (FAs) from adipose tissue or diet are processed in the liver into TGs, which are then complexed with phospholipid and cholesterol to be exported as lipoproteins. Accumulation occurs if there is increased mobilization of adipose tissue (starvation, diabetes), over-activity of enzymes (alcohol consumption), decreased TG oxidation, or decreased apo-protein synthesis. A fatty liver appears pale, enlarged, friable, and greasy. Iron deposition, or hemosiderin, occurs locally at hemorrhage sites as hemoglobin breaks down or systemically in hemochromatosis, an inherited defect where toxic free ferric iron produces radicals that harm tissues. Bilirubin accumulation, known as jaundice or icterus, results in a yellow pigmentation of the skin and eyes. Jaundice is caused by hemolysis (overproduction of unconjugated bilirubin), hepatocellular dysfunction (liver injury affecting uptake), or bile flow obstruction (cholestasis). While often just a discoloration, high levels can lead to toxic injury in hepatocytes or neuronal dysfunction (kernicterus) in the brain.

Genetic abnormalities also cause cellular decline. Inherited or acquired somatic mutations (caused by radiation, drugs, or viruses) do not pass to offspring but affect the individual. These abnormalities interfere with mitosis, causing anemia or intestinal dysfunction, fail to synthesize vital structural proteins leading to necrosis, or fail to synthesize enzymes, resulting in congenital disease.

Recognizing and Clinically Assessing Necrosis

Necrosis is defined as the point of no return for cell injury. It results from enzymatic digestion and protein denaturation, with changes typically becoming visible via light microscopy after approximately 6 to 8 hours. Evidence of necrosis is categorized as gross, cytoplasmic, or nuclear. Gross evidence includes patterns like coagulation necrosis, liquefaction necrosis, caseation necrosis, and fat necrosis. Fat necrosis is subdivided into enzymatic (pancreatic lipase breaking down TGs into glycerol and FAs, which complex with calcium to form white, chalky soaps) and non-enzymatic (trauma-induced, often leading to a granulomatous inflammatory response).

At the microscopic level, cytoplasmic evidence includes the cytoplasm becoming more homogenous and deeply staining (pinker with H&E stains) due to protein denaturation and ribosome loss. It may also look vacuolated due to mitochondrial swelling. Dystrophic calcification, the deposition of calcium salts in dead tissues despite normal serum calcium levels, often accompanies necrosis. The suicide bag hypothesis once proposed that lysosomal enzyme release caused death, but it is now understood that this release is a result of death rather than the cause. Nuclear evidence provides a more definitive indication of necrosis. The nuclear chromatin clumps, and the nucleus becomes smaller and darker, a process called pyknosis. This is followed by karyorrhexis (fragmentation) or karyolysis (complete lysis via enzymes).

Clinical problems associated with necrosis include altered function, tissue loss, secondary infections, and systemic effects. For example, a myocardial infarction (heart attack) involves the occlusion of blood supply to the heart, causing focal necrosis that leads to chest pain, dyspnea, and potential circulatory failure. Small areas of brain necrosis (stroke) can have devastating consequences for motor function. Tissue loss is exemplified by gangrene, which follows vascular patterns; frostbite typically affects distal areas like fingers and ears. Dead tissue is darkly discolored and clearly demarcated unless secondary bacterial infections cause liquefaction and foul odors. Certain bacteria like C. perfringens produce gas within necrotic tissues. Secondary infections thrive in necrotic areas because they are beyond the reach of the immune system. Systemic effects include fever (pyrogen release) and increased white blood cell counts. Local effects include gastric ulceration, which may lead to melena (dark, tarry stools) and anemia. Diagnostically, the release of cytoplasmic enzymes into the blood allows clinicians to assay specific markers for liver or heart damage.

Programmed Cell Death and Post-Mortem Changes

Apoptosis, or cellular suicide, is an active, programmed form of cell death essential for development and physiology. It occurs during embryogenesis, hormonally-driven regression (uterine endometrium), and in high-turnover tissues like skin or the intestine. It also eliminates self-reactive lymphocytes. Pathologic apoptosis is triggered by DNA damage, misfolded proteins, viral infections, or atrophy. Cytotoxic T-cells can also induce apoptosis in abnormal neighboring cells. Unlike necrosis, apoptosis requires energy and follows a plan mediated by caspases. These enzymes form a cascade leading to “executioner caspases” that degrade DNA and proteins. This results in neatly packaged fragments cleared by macrophages without inducing inflammation. Autophagy is a related process where a cell “eats” its own non-essential organelles to provide energy during depletion; if nutrients are not restored, it can lead to apoptosis.

Following somatic death, the body undergoes several recognizable changes. Rigor mortis is the stiffening of muscles due to ATP reduction. Post-mortem lividity occurs as blood settles gravitationally, leading to green skin discoloration from hemoglobin breakdown. Large post-mortem blood clots may form in the heart. Putrefaction involves fermentation by saprophytic bacteria, producing gas that can rupture the stomach or cause a “foamy” liver. Autolysis is the gross disintegration of tissues by their own enzymes. While microscopically similar to necrosis due to lack of ATP, autolysis is generally not associated with an inflammatory response.

Fluid Dynamics and the Development of Edema

Normal fluid exchange is governed by the balance of vascular hydrostatic pressure and oncotic pressure. Osmosis moves water toward higher solute concentrations. Oncotic pressure is the osmotic pull exerted by plasma proteins, primarily albumin, while hydrostatic pressure is the physical pressure of the fluid influenced by heart activity and vessel quantity. At the arteriolar end of a capillary, hydrostatic pressure exceeds oncotic pressure, driving fluid and nutrients into tissues. At the venular end, oncotic pressure exceeds hydrostatic pressure, pulling waste-laden fluid back into the vessels. Fluid that remains in the tissue spaces, the ultrafiltrate, is drained by the lymphatic system.

Edema is the excess accumulation of fluid in extracellular spaces. Localized edema (e.g., a bug bite) results from a focal disturbance, such as venous or lymphatic obstruction. If focal pressure leaves a depression, it is called pitting edema. Chronic edema can lead to tissue fibrosis, making the area thick and firm. Generalized edema occurs throughout the body and is often seen in congestive heart failure. Effusions are accumulations in body cavities, such as hydrothorax (pleural space), hydropericardium (pericardial sac), and ascites (peritoneal cavity). Anasarca describes massive whole-body edema.

In right-sided heart failure, blood backs up in the systemic circulation, increasing venous hydrostatic pressure and causing generalized edema. In left-sided heart failure, blood accumulates in pulmonary circulation, leading to increased pulmonary venous hydrostatic pressure. Fluid moves into the alveolar spaces (pulmonary edema), causing dyspnea (difficulty breathing) and orthopnea (difficulty breathing when lying down). Heart failure also decreases blood flow to the kidneys, triggering the Renin-Angiotensin-Aldosterone system. Renin stimulates the production of Angiotensin II, which triggers the adrenal cortex to release aldosterone. Aldosterone promotes sodium and water retention, increasing blood volume and decreasing vascular oncotic pressure through dilution. Hypoproteinemia, specifically hypoalbuminemia (low albumin), is the second major cause of generalized edema. For instance, in protein-losing nephropathy, albumin is lost in the urine (proteinuria), lowering plasma oncotic pressure and causing fluid to remain in tissues, often manifesting as ascites. This triggers the same renal compensation, worsening the edema via dilution.

Hemostasis and Hemorrhage

Hemorrhage is the presence of blood in interstitial tissues, body cavities, or externally. It can result from vessel injury or blood cells squeezing through intact endothelial gaps. Free red blood cells are broken down into hemosiderin. Clinical manifestations in the skin include petechiae (pinpoint), purpura (3-5 mm), ecchymoses (bruises), and hematomas (large pools). Hemopericardium is particularly dangerous; if blood pressure in the pericardial sac overcomes the pressure in the right atrium and ventricle, it causes cardiac collapse. Other terms include hyphema (eye bleeding), epistaxis (nosebleed), and hemoptysis (coughing up blood).

Normal hemostasis maintains blood in a fluid state while allowing for the rapid arrest of bleeding (clotting) at injury sites. It involves transient arteriolar vasoconstriction, primary hemostasis (platelet plug), secondary hemostasis (fibrin clot via the coagulation cascade), and a permanent plug with concurrent fibrinolysis. Platelets are non-nucleated discs that aggregate using fibrinogen to form a plug. Their function is modulated by prostaglandins like PGI (anti-clotting, from endothelium) and thromboxane A (pro-clotting, from platelets). The intact endothelium is antithrombotic, but once disrupted, it exposes collagen and tissue factor, becoming prothrombotic.

The coagulation cascade is a series of enzymatic conversions. Tissue factor is the main initiator in vivo. The process requires calcium ions, a phospholipid surface, and Vitamin K (essential for synthesizing prothrombin and factors VII, IX, and X). The cascade culminates in the conversion of prothrombin to thrombin, which transforms fibrinogen into insoluble strands of fibrin. Fibrinolysis occurs simultaneously to limit clot size. Plasminogen is converted to plasmin (fibrinolysin), which breaks down fibrin. Tissue-type plasminogen activator (tPA) is a therapeutic agent used to manage thrombosis.

Abnormal hemostasis includes coagulation disorders like Hemophilia A (X-linked deficiency of Factor VIII) and von Willebrand’s Disease (autosomal dominant deficiency of vWF, affecting both platelet adhesion and Factor VIII levels). Increased anticoagulant activity, such as accidental ingestion of warfarin (rodenticide) or sweet clover poisoning in cattle (Vitamin K antagonist), also causes hemorrhage. Platelet disorders include thrombocytopenia (low numbers) and thrombocytopathia (functional defects, sometimes induced by Aspirin).

Thrombosis, Ischemia, and Infarction

Thrombosis is inappropriate clotting within a vessel, governed by Virchow’s Triad: endothelial injury, alterations in blood flow, and hypercoagulability. Endothelial injury can be acute (trauma, inflammation) or chronic (atherosclerosis). Altered flow includes turbulence (fast, erratic) and stasis (pooling), both of which promote interaction with the endothelium. Hypercoagulability involves an imbalance in clotting/fibrinolysis and can be inherited (Factor V mutations) or acquired, such as through polycythemia (increased RBC count), which increases blood viscosity. "Economy Class Syndrome" refers to deep vein thrombosis in passengers on long flights due to inactivity, high altitude, and stasis.

Ischemia is the reduction of blood supply to tissues, leading to hypoxia. It can cause functional changes (memory loss in the brain) or pain (angina/claudication). An infarct is the localized area of necrosis resulting from ischemia. Factors influencing the outcome include the availability of collateral circulation, tissue susceptibility (brain and heart are highly susceptible), metabolic rate, and the rate of obstruction development. In cats, feline cardiomyopathy often leads to thrombi in the left atrium that embolize to the aortic bifurcation, causing hind limb paralysis and pain. An embolus is a mass (solid, gas, or liquid) carried by the blood to a site distant from its origin. Arterial infarcts in dense tissues like the kidney or heart are often pale, while those in loose tissues or organs with dual blood supplies (lungs, liver) are hemorrhagic.

Venous obstruction increases hydrostatic pressure, causing edema or, if severe, capillary rupture and hemorrhage (red infarct). This is seen in mesenteric torsion in horses, where a twist in the colon root collapses thin-walled veins, leading to venous infarction and shock. Reperfusion injury occurs when restoring blood supply to an ischemic area exacerbates damage through oxidative stress and the release of inflammatory mediators (ROS) into the bloodstream.

Disseminated Intravascular Coagulation (DIC) and Shock

DIC is a thrombohemorrhagic syndrome resulting from widespread coagulation activation. It is a secondary complication triggered by tissue factor release or widespread endothelial injury (sepsis, trauma, burns). Initially, it causes microvascular thrombosis and organ damage (hypoxia). As platelets and clotting factors are consumed, a hemorrhagic phase begins. The fibrinolysis cascade also activates, producing fibrin breakdown products with anticoagulant properties. DIC is diagnosed via evidence of damaged RBCs, thrombocytopenia, and clinical signs of both clotting and bleeding.

Shock is the reduction in tissue perfusion due to decreased cardiac output or effective circulating blood volume. Types include hypovolemic (fluid/blood loss), septic (vasodilation and SIRS from microbial infection), cardiogenic (heart pump failure), neurogenic (loss of vascular tone from spinal injury), and anaphylactic (IgE-mediated hypersensitivity). In the non-progressive (compensated) stage, the body maintains pressure via increased heart rate and peripheral vasoconstriction (pallor and cool skin). Renal fluid retention also occurs. In the progressive stage, widespread tissue hypoxia leads to metabolic acidosis, further cell injury, and multiple organ dysfunction syndrome (ARDS, kidney failure, intestinal necrosis). Perfusing the brain and heart becomes impossible, leading to death.

The Acute Inflammatory Response

Acute inflammation is the first line of defense, marked by redness, heat, swelling, pain, and loss of function. It involves vascular changes and a cellular response. Arterioles first undergo transient constriction, followed by vasodilation (hyperemia) to increase blood flow. Vasoactive amines like histamine (from mast cells) and kinins facilitate this. Capillaries and venules become leaky, allowing fluid and proteins to move into extravascular space (exudation), causing localized edema. Increased permeability is caused by endothelial cell contraction (histamine-mediated), retraction (TNF and IL-1 mediated), or direct necrosis.

Exudates are protein-rich fluids (with immunoglobulins, complement, and fibrinogen) that form due to increased permeability. They have a higher specific gravity than transudates, which are ultrafiltrates formed under normal permeability (e.g., in heart failure). Fibrin, a polymer formed from fibrinogen by Factor XIII, helps localize the inflammatory process and provides a scaffold for movement. In cattle, shipping fever caused by Mannheimia hemolytica is characterized by abundant fibrin on the lungs and pleura.

Leukocyte recruitment involves several steps. As flow slows, leukocytes move toward the endothelium (margination) and transiently stick (rolling) via selectins (L-selectins and P-selectins). Firm adhesion is mediated by integrins. Leukocytes then squeeze through junctions (diapedesis/transmigration) into the interstitium, a process facilitated by PECAM-1. Chemotaxis is the directional migration of cells along a chemical gradient (C5a, leukotrienes). Leukocytes are activated via Toll-like receptors that recognize Pathogen-Associated Molecular Patterns (PAMPs) on microbes. Phagocytosis follows: cells (neutrophils and macrophages) recognize pathogens, often coated in opsonins (IgG, C3b), engulf them into a phagosome, and destroy them using Reactive Oxygen Species (ROS) and lysosomal acid hydrolases.

Mediators and Clinical Aspects of Inflammation

Inflammatory mediators are derived from plasma (circulating as inactive precursors) or cells (pre-formed in granules). Key groups include vasoactive amines (histamine, serotonin), plasma proteases (kinin system, coagulation cascade, complement system), and lipid-derived eicosanoids (prostaglandins and leukotrienes). Factor XII (Hageman factor) is a central trigger for the kinin, clotting, and complement systems. Cytokines, especially IL-1 and TNF, modulate the function of other cells and induce systemic acute-phase responses.

Common anti-inflammatory drugs include NSAIDs (Aspirin, Ibuprofen), which inhibit the conversion of arachidonic acid to prostaglandins, and corticosteroids (Prednisone), which block the initial conversion of phospholipids to arachidonic acid and stabilize lysosomal membranes. Systemic signs of inflammation, known as the acute phase reaction, include fever and changes in white blood cell counts. Fever is caused by pyrogens (IL-1, TNF) acting on the hypothalamus via prostaglandins. Leukocytosis (increased WBC) often features a "left shift," meaning an increase in immature neutrophils in the blood due to heavy demand. Viral infections may cause leukopenia or lymphocytosis. Increased plasma proteins like C-reactive protein and fibrinogen also indicate inflammation.

Acute inflammation can lead to resolution, where macrophages clear debris and the tissue returns to normal. Alternatively, suppurative (purulent) inflammation occurs if bacteria cause liquefactive necrosis and pus formation (a mix of dead neutrophils and tissue). An abscess is a walled-off area of pus. Sepsis occurs when bacteria or toxins spread through the bloodstream, potentially causing shock or DIC. Bacteremia is more transient, though it can colonize damaged heart valves.

Chronic and Granulomatous Inflammation

Chronic inflammation is marked by the presence of mononuclear cells (lymphocytes, plasma cells, macrophages) and occurs when an injurious agent persists. It involves ongoing tissue injury, an immune response, and healing through granulation tissue and fibrosis (angiogenesis and collagen deposition). Granulomatous inflammation is a specific form characterized by epithelioid cells—activated macrophages with foamy, pale cytoplasm that excel at secretion but are less efficient at phagocytosis. This requires a T-lymphocyte-mediated response. Tuberculosis is the classic example, forming tubercles. Other causes include leprosy, syphilis, fungal infections (Blastomycosis), parasites, and foreign bodies like sutures. Foreign body granulomas occur when the material is too large to be engulfed by a single cell.

Non-granulomatous chronic inflammation involves scattered lymphocytes and plasma cells and is seen in chronic viral infections, autoimmune diseases, or chronic toxic diseases (alcoholism). Chronic suppurative inflammation results when the body cannot clear a pyogenic stimulus, leading to abscesses or osteomyelitis (bone infection). Bone necrosis can create a sequestrum, a devitalized bone fragment. Chronic inflammation can also cause amyloidosis, the deposition of abnormally folded insoluble proteins in the interstitium. Common forms include AL amyloid (from plasma cell tumors), AA amyloid (reactive, from chronic inflammation), and Ab (associated with Alzheimer's disease). Amyloid makes tissues firm and enlarged, potentially interfering with nutrient diffusion.

The Adaptive Immune Response

The immune response is characterized by specificity, memory, and amplification. Antigens are molecules (usually proteins or polysaccharides) that evoke a response. Haptens are small molecules that become antigenic only when complexed with a carrier. Natural tolerance prevents the body from attacking its own tissues. The immune response includes cell-mediated immunity (T cells) and humoral immunity (B cells/antibodies). T cells develop in the thymus, while B cells develop in the bone marrow. Both types migrate to peripheral tissues like lymph nodes or the spleen.

B cells have surface receptors (BCRs). Upon contact with an antigen, they undergo clonal expansion and differentiate into plasma cells (which secrete antibodies) and memory cells (which provide long-term immunity). Antibodies (immunoglobulins) include IgG (crosses the placenta), IgM (first produced in the primary response), IgA (mucosal), IgE (parasites/allergies), and IgD. T cells (TCRs) interact with antigen fragments presented on cells. CD4+ helper T cells (Th1 and Th2) secrete cytokines to regulate the response, while CD8+ cytotoxic T cells directly kill virus-infected or tumor cells. Natural Killer (NK) cells provide early innate defense by killing stressed cells lacking MHC Class I molecules. Antigen-presenting cells (macrophages and dendritic cells) process antigens and present them to lymphocytes.

Primary immune response follows the first exposure and has a lag period with initial IgM production. Secondary (anamnestic) response is much faster and higher in IgG. Passive immunity is the transfer of pre-formed antibodies (transplacental, colostral, or therapeutic antivenins) and is temporary. Active immunity develops through infection or vaccination, where killed or attenuated organisms stimulate a primary exposure response without pathogenicity. Serology measures antibody titers; a high titer indicates high antibody levels.

Hypersensitivity and Immunodeficiency

Hypersensitivity reactions occur when an immune response causes tissue injury. Type I (Immediate) is IgE-mediated, involves mast cell degranulation (histamine), and causes allergies or life-threatening anaphylaxis. Type II (Antibody-mediated) involves IgG or IgM binding to cell surface antigens, leading to opsonization and phagocytosis (autoimmune hemolytic anemia) or cellular dysfunction (Myasthenia gravis, Graves’ disease). Type III (Immune complex-mediated) involves circulating Ag-Ab complexes deposited in vessel walls, causing vasculitis and fibrinoid necrosis (Serum sickness, Arthus reaction, Glomerulonephritis). Type IV (Cell-mediated) is a delayed response involving sensitized T cells that are either cytotoxic or secrete cytokines to activate macrophages (contact dermatitis, TB tests).

Immunodeficiency can be primary (congenital, like SCID) or secondary (acquired via malnutrition, cancer, or infection). AIDS is the most significant secondary immunodeficiency, caused by the HIV retrovirus. HIV infects CD4+ T cells, macrophages, and dendritic cells by binding to CD120 and co-receptors. Once the virus is integrated as pro-viral DNA, cell activation (via other infections) triggers viral replication and CD4+ cell death. This leads to profound immunosuppression, opportunistic infections (fungi, protozoa), and cancers like Kaposi’s sarcoma. The disease progresses from an acute phase to a long chronic phase, ending in a final crisis with high viremia and low CD4+ counts.

Tissue Repair, Regeneration, and Fibrosis

Haling involves regeneration (replacing lost cells with the same type) or scarring (fibrosis). Labile cells (skin, bone marrow) divide constantly. Stable cells (liver, kidney, fibroblasts) have a low division rate but can multiply if the connective tissue framework is intact. Permanent cells (neurons, cardiac muscle) cannot divide; thus, injury results only in scarring. Rate of proliferation is controlled by cyclins and growth factors. Regeneration is preferred, as seen in the recovery of intestinal enterocytes after rotavirus (villus atrophy), provided crypt stem cells survive. If injury is severe or affects permanent cells, healing occurs via fibrosis.

Scarring follows several stages: inflammation and debridement (neutrophils and macrophages clear debris), granulation tissue formation (immature connective tissue with fibroblasts and new capillaries), angiogenesis (new blood vessel growth), collagenization (fibroblasts laying down procollagen), and maturation/remodeling (granulation tissue replaced by mature Type I collagen). Scar strength increases with cross-linking, and the size decreases through contraction mediated by myofibroblasts. Skin injuries like abrasions (scrapes) heal by regeneration, whereas lacerations (tears) and incisions (surgical cuts) may require healing by first intention (edges approximated by sutures) or second intention (larger defects requiring more granulation tissue). Keloids are abnormal nodular masses of collagen that can follow even minor wounds. Factors like Vitamin C deficiency, corticosteroids, and diabetes-related poor blood supply delay healing.

Infection and Infectious Agents

Infectious agents range from prions (misfolded proteins) and viruses (DNA/RNA) to bacteria, fungi, and parasites (protozoa/helminths). Organisms enter through primary portals like mucosal barriers or via direct inoculation. Spread occurs through physical contact, airborne dust/droplets, food/water, or insect vectors. Virulence, dose, and host immune status determine the outcome. Infection may be subclinical (no symptoms but an immune response), localized, or systemic. Bloodstream infections are viremia, bacteremia, fungemia, or parasitemia. Sepsis (septicemia) involves clinical systemic illness from bacteria or toxins.

Viruses use host machinery for replication and are categorized by their cytopathic effects (cell death) or ability to transform cells (cancer). Herpes viruses cause latent infections, remaining dormant in the host for life. Bacteria produce injury via endotoxins (LPS of Gram-negative cell walls, causing fever and shock) or exotoxins (secreted proteins). Locally acting exotoxins like coagulase (Staphylococci) or gas-producing enzymes (C. perfringens) aid invasion. Remotely acting exotoxins cause diseases like Tetanus (muscle spasms) or Botulism (flaccid paralysis). Hamburger disease (E. coli O157:H7) involves a verocytotoxin that causes hemorrhagic colitis and hemolytic uremic syndrome. Fungal infections are often opportunistic in immunosuppressed patients and can be superficial (ringworm, yeast infections) or systemic (histoplasmosis).

Parasites include one-celled protozoa (Toxoplasmosis, which can be congenital) and multicellular helminths like nematodes (Hookworms, Roundworms), cestodes (Tapeworms), and trematodes (Flukes). Ectoparasites like fleas cause hypersensitivity and carry disease as mechanical or biological vectors.

Neoplasia and the Biological Basis of Cancer

Neoplasia is disorganized tissue growth unresponsive to normal controls, resulting in a tumor (neoplasm). It involves non-lethal genetic damage to growth-promoting proto-oncogenes, growth-inhibiting tumor suppressor genes (RB, p53), or genes regulating apoptosis and DNA repair. Cancer cells demonstrate hallmarks: self-sufficiency in growth signals (mutating into oncogenes), insensitivity to inhibitory signals, evasion of cell death, limitless replicative potential (telomerase activation), sustained angiogenesis, and the ability to invade and metastasize. Carcinogens include chemicals (tar, epoxides), radiation (UV light), and viruses (HPV, HBV, retroviruses like FeLV).

Tumors are classified as epithelial (adenoma/carcinoma) or mesenchymal (fibroma/fibrosarcoma). Benign tumors are well-differentiated, grow slowly, and are often encapsulated. Malignant tumors show anaplasia (lack of differentiation), pleomorphism (variation in size and shape), and high nuclear-to-cytoplasmic ratios. They are invasive and metastasize via lymphatics or the bloodstream. Paraneoplastic syndromes are effects of cancer unrelated to direct tissue involvement, like cachexia (weight loss/wasting), hypercalcemia, or hormonal secretions. Diagnosis involves history, imaging, biopsy, and exfoliative cytology. Staging (TNM system) determines the extent of spread, which informs therapy (surgery, radiation, chemotherapy, or immunotherapy). Outcomes depend on the specific tumor type and the stage at time of diagnosis.