Chapter 21

Concurrent Immune Defenses and Complement System Pathways

• Integration of Immune Defenses:

  • Immune defenses do not operate sequentially (first line, then second line, then third line in isolated stages).

  • Upon entry or contact of a pathogen with the body, all immune systems activate simultaneously.

  • Pathogen contact triggering the complement system concurrently activates humoral immunity; B cells begin differentiating into plasma cells to synthesize antibodies against the foreign antigen.

• The Complement System:

  • A group of antimicrobial proteins synthesized primarily by the liver.

  • Circulate in inactive forms and undergo a complex cascade of activation upon encountering pathogens.

• Pathways of Complement Activation:

  • Classical Pathway:

    • Antibody-dependent.

    • Requires an antibody bound to an antigen on the surface of a pathogen to initiate the cascade.

  • Alternative Pathway:

    • Antibody-independent and spontaneously active.

    • Complement protein C3 is continuously cleaved into C3a and C3b at low levels.

    • C3b binds directly to carbohydrates and surface molecules on microbes, activating the cascade.

  • Lectin Pathway:

    • Antibody-independent.

    • Involves lectin proteins, such as mannose-binding lectin.

    • Lectins bind to specific surface carbohydrates (such as mannose sugars) on microbes to trigger the cascade.

Complement System Split Functions and Effector Mechanisms

• Cleavage of C3:

  • All three complement pathways converge on the cleavage of the C3 protein into two distinct fragments: C3a and C3b.

• Functions of C3a:

  • Basophil and Mast Cell Activation:

    • Binds to basophils and mast cells, causing them to degranulate and release inflammatory toxins.

    • Secretes histamine and other inflammatory mediators.

    • Histamine acts as a vasodilator, dilating blood vessels to increase local blood flow and facilitate inflammation and immune cell recruitment.

  • Phagocyte Stimulation:

    • Stimulates neutrophils and macrophages (phagocytic white blood cells) to promote pathogen ingestion and tissue clearance.

  • Inflammatory and Chemotactic Activity:

    • Acts alongside C5a as a chemotoxin (chemotactic agent) and an anaphylatoxin to attract leukocytes and amplify the immune response.

• Functions of C3b:

  • Immune Clearance:

    • Binds to antigen-antibody complexes.

    • Attaches these complexes to red blood cells (RBCs).

    • RBCs transport the complexes to the liver and spleen, where resident macrophages remove and phagocytize them, clearing immune complexes from circulation.

  • Opsonization:

    • Binds directly to surfaces of bacteria, viruses, and other pathogens.

    • Serves as molecular handles ("forks on a meatball") that phagocytes grab to enhance ingestion.

    • Opsonization is defined strictly as enhanced phagocytosis.

  • Cytolysis (Membrane Attack Complex / MAC):

    • Binds to pathogens and recruits subsequent complement proteins (including C5b, C6, C7, C8, and C9) to form the Membrane Attack Complex (MAC).

    • The MAC drills a pore through the target cell membrane.

    • Water rushes into the cell while salts spill out, causing osmotic lysis (cytolysis / cell popping).

Natural Killer Cells and Neutrophil Chemotaxis

• Neutrophil Chemotaxis Mechanism:

  • Chemotaxis is the directed movement of white blood cells along a chemical concentration gradient (sniffing out chemical signals like a bloodhound).

  • Neutrophils sense chemoattractants (such as C5a or C3b) via membrane receptors.

  • Upon sensing a gradient, neutrophils polarize, send out cellular protrusions (pseudopodia), and reorient their cell bodies toward the chemical source.

  • If the source moves, the neutrophil immediately reorients toward the new position.

• Natural Killer (NK) Cells:

  • A specialized class of cytotoxic lymphocytes that continuously patrol the body (immune surveillance).

  • Target diseased host cells, particularly virus-infected cells and cancerous cells (such as neuroblastoma tumor cells).

• NK Cell Effector Mechanisms:

  • Perforins:

    • Proteins secreted by NK cells that bind to the target cell membrane and polymerize to form ring-like pores (perforations).

    • Though smaller than the MAC, perforin pores breach the membrane barrier.

  • Granzymes:

    • Protein-degrading enzymes secreted by NK cells that enter the target cell through the perforin pores.

    • Once inside, granzymes destroy internal cellular proteins and activate enzymes that induce apoptosis.

  • Apoptosis: Programmed cell death (cellular suicide).

  • After the target cell undergoes apoptosis and fragmenting, macrophages arrive to phagocytize the remaining cellular debris.

Apoptosis Mechanisms and Cellular Breakdown

• Molecular Cascade of Apoptosis:

  • Initiated by extracellular signals (e.g., NK cells or cytotoxic Killer T cells binding to specific cell-surface receptors) or intracellular stress.

  • Binding activates adapter proteins inside the host cell.

  • Adapter proteins activate proteolytic enzymes (caspases) that break down structural and functional internal proteins.

  • Proteolytic activity targets the mitochondria, causing the electron transport chain to collapse.

  • Disruption of the electron transport chain halts ATP production, depriving the cell of metabolic energy.

  • Enzymatic cleavage dismantles the cytoskeleton, leading to structural breakdown, membrane blebbing, and cellular fragmentation into nonfunctional membrane-bound bodies.

  • Resident or recruited macrophages phagocytize these apoptotic fragments without triggering widespread tissue damage.

Pyrexia (Fever) Physiology, Pyrogens, and Hypothalamic Regulation

• Pyrexia (Fever):

  • An abnormal elevation of core body temperature.

  • Triggers include trauma, systemic infections, drug reactions, and brain tumors.

  • Normal average body temperature is approximately $98.6^\circ\text{F}$.

  • A low-grade fever is typically recognized around $100.3^\circ\text{F}$ to $100.4^\circ\text{F}$ ($38^\circ\text{C}$).

  • High, dangerous fevers reach $102^\circ\text{F}$ to $103^\circ\text{F}$ or higher, which can cause protein denaturation, cellular damage, and brain damage.

• Pyrogens:

  • Chemical agents that stimulate heat production and elevate the body's thermal set point.

  • Exogenous Pyrogens: Originate outside the body (e.g., surface proteins, lipopolysaccharides, or endotoxins from bacteria and viruses).

  • Endogenous Pyrogens: Secreted by internal immune cells (e.g., interleukin-1, tumor necrosis factor, and cytokines released by neutrophils and macrophages) or tumor cells.

• Hypothalamic Set-Point Regulation:

  • The body's central thermostat is located in the anterior hypothalamus.

  • Pyrogens induce the anterior hypothalamus to synthesize and secrete Prostaglandin E2 ($\text{PGE}_2$).

  • $\text{PGE}_2$ acts directly on hypothalamic neurons to raise the body's thermal set point.

• Physiological Benefits of Low-Grade Fever:

  • Accelerates metabolic rate and chemical reactions, speeding up immune defenses, antibody production, and tissue repair.

  • Enhances the antimicrobial activity of interferons.

  • Inhibits the growth and reproduction of temperature-sensitive bacteria and viruses.

  • Enforces physical rest by inducing fatigue and lethargy, conserving metabolic energy for immune defense.

• Antipyretics:

  • Medications (e.g., acetaminophen/Tylenol, ibuprofen) that inhibit prostaglandin synthesis to reduce fever.

  • Suppressing a low-grade fever can slow down immune clearance and prolong recovery times.

  • Steroids similarly suppress global immune responses and fever mechanisms.

Inflammation Fundamentals and the Cardinal Signs

• Inflammation:

  • A localized defensive response to tissue injury, trauma, or infection.

  • Primary Objectives:

    1. Contain and prevent the spread of pathogens.

    2. Destroy invading pathogens and remove necrotic tissue debris.

    3. Initiate tissue repair and regeneration.

• Four Cardinal Signs of Inflammation:

  • Redness (Rubor):

    • Caused by local hyperemia (increased blood flow) following arterial vasodilation.

  • Heat (Calor):

    • Caused by local hyperemia delivering warm systemic blood to the superficial tissues.

  • Swelling (Tumor):

    • Caused by increased capillary permeability, allowing fluid, proteins, and cells to filter out of capillaries into tissue spaces (Edema).

  • Pain (Dolor):

    • Caused by direct mechanical injury to nerves, pressure on nociceptors from edematous swelling, and chemical stimulation of nerve endings by inflammatory cytokines and bacterial toxins.

• Fifth Potential Sign:

  • Loss of Function (Functio laesa):

    • Results from severe pain and structural swelling, preventing movement to protect the injured region.

Cytokines and Leukocyte Mobilization Processes

• Cytokines:

  • Small protein signaling molecules secreted by immune and tissue cells to mediate intercellular communication and coordinate immune responses.

  • Examples: Interferons, interleukins, tumor necrosis factors (TNFs), chemotactic factors, histamine, and leukotrienes.

  • Sources: Macrophages, lymphocytes, damaged endothelial cells, fibroblasts, mast cells, and granulocytes (neutrophils, eosinophils, basophils).

• Chemical Mediators of Vasodilation and Permeability:

  • Histamine, leukotrienes, and other cytokines induce local vasodilation, generating hyperemia.

  • These mediators increase capillary permeability, creating widened intercellular clefts through which fluid, plasma proteins, and leukocytes exit.

• Leukocyte Mobilization Steps:

  1. Vessel Changes: Vasodilation causes local blood flow velocity to slow down dramatically within the leaky capillaries.

  2. Margination: Neutrophils circulating in the bloodstream slow down, roll along the endothelial wall, and adhere tightly to cell adhesion molecules displayed on damaged endothelial cells.

  3. Diapedesis (Extravasation):

     • Adhered neutrophils change shape and actively squeeze through narrow pores between endothelial cells into the interstitial tissue spaces.

  4. Chemotaxis:

     • Extravasated neutrophils navigate through the tissue matrix toward higher concentrations of chemoattractants secreted at the site of injury.

  5. Phagocytosis:

     • Neutrophils engulf and destroy foreign bacteria and cellular debris via phagocytosis.

Tissue Cleanup, Pus Formation, and Regeneration

• Pathogen Containment Mechanisms:

  • Fibrinogen:

    • Filters out of permeable capillaries into the tissue fluid where it converts to fibrin.

    • Forms a sticky, gel-like mesh network that walls off microbes, preventing systemic dispersal.

  • Heparin:

    • An anticoagulant secreted by mast cells and basophils in the immediate center of the injury site.

    • Prevents blood clotting within the isolated central pocket, keeping liquid corridors open for white blood cells, complement, and antibodies to maneuver.

• Tissue Cleanup Process:

  • Neutrophils: Arrive first at the injury site within 1 hour to combat microbes.

  • Monocytes: Arrive within 8 to 12 hours and transform into mobile, highly phagocytic macrophages.

  • Macrophage Functions: Engulf remaining bacteria, damaged host cells, and dead or dying neutrophils.

  • Role of Edema in Cleanup: Swelling compresses surrounding venules, reducing venous drainage while forcing open the one-way flap valves of lymphatic capillaries. This promotes lymphatic drainage of fluid, dead cells, and cellular debris.

• Pus and Abscess Formation:

  • Pus: A viscous, yellowish fluid accumulation composed of dead neutrophils, degraded tissue cells, living and dead bacteria, and interstitial fluid.

  • Abscess: A localized collection of pus contained within a tissue cavity, often surrounded by a fibrous fibrin wall synthesized by the body to isolate the infection.

• Tissue Repair Mechanisms:

  • Blood platelets and damaged endothelial cells release Platelet-Derived Growth Factor (PDGF) and other growth factors.

  • PDGF stimulates local fibroblasts to divide rapidly and synthesize collagen matrix.

  • Hyperemia supplies essential oxygen, amino acids, and nutrients required for protein synthesis.

  • Increased local temperature accelerates cellular metabolic rate and speeds up mitosis.

  • The fibrin clot acts as a structural scaffold for tissue reconstruction.

  • Pain forces rest of the affected body part to prevent further structural damage during healing.

Adaptive Immunity Characteristics and Branches

• Adaptive Immunity (Third Line of Defense):

  • Acquired immune responses characterized by three core features:

    1. Systemic Effect: Operates throughout the entire body rather than being restricted to a localized site of infection.

    2. Specificity: Directs custom responses against specific target antigens on an individual basis, determined by genetic makeup and specific lifetime pathogen exposures.

    3. Memory: Retains long-lasting memory of previously encountered antigens. Upon re-exposure, memory cells initiate a rapid, potent secondary response that eliminates the pathogen before noticeable clinical illness occurs (the foundational principle of vaccination).

• Two Major Branches of Adaptive Immunity:

  • Cell-Mediated Immunity (T Lymphocytes / T Cells):

    • Directly attacks and destroys infected host cells, foreign cells, or abnormal cells.

    • Clears intracellular pathogens (viruses, intracellular bacteria) hiding inside human cells where antibodies cannot penetrate.

    • Attacks parasitic worms, cancer cells, and transplanted foreign tissues.

  • Humoral Immunity / Antibody-Mediated Immunity (B Lymphocytes / B Cells):

    • B cells differentiate into specialized plasma cells, which synthesize and secrete antibodies.

    • Antibodies do not directly kill pathogens; instead, they bind specifically to antigens and tag them for destruction by phagocytes, complement proteins, or NK cells.

    • Targets extracellular pathogens floating in body fluids (extracellular viruses, bacteria, yeast, protozoans) and noncellular molecular antigens (bacterial toxins, venoms, allergens).

Classifications of Active and Passive Immunity

• Active Immunity:

  • Occurs when an individual's own T cells or B cells are stimulated by an antigen, resulting in a full adaptive immune response and the formation of long-term immune memory.

  • Natural Active Immunity:

    • Production of one's own antibodies or T cells as a result of natural exposure to an infectious pathogen during an illness.

  • Artificial Active Immunity:

    • Production of one's own antibodies or T cells as a result of vaccination/immunization.

    • Involves introducing a weakened, killed, or fragment antigen artificially (from a laboratory) to induce memory without causing disease.

• Passive Immunity:

  • Occurs when an individual receives preformed antibodies produced by another person or animal.

  • The recipient's own immune cells are not stimulated, and no immune memory is produced; protection lasts only as long as the transferred antibodies persist in the body.

  • Natural Passive Immunity:

    • Temporary acquisition of antibodies naturally transferred from mother to offspring.

    • Examples: Maternal IgG antibodies crossing the placenta into fetal circulation, or IgA antibodies transferred to an infant through breast milk during breastfeeding.

  • Artificial Passive Immunity:

    • Temporary protection resulting from the therapeutic injection of immune serum or antibodies obtained from another human or animal source.

    • Examples: Injections of anti-venom (for snakebites), anti-toxins, or specific immunoglobulin therapies.