Normal Immune Responses: Comprehensive Study Notes

Anatomy and Structural Organization of the Immune System

• External Defenses:

  • Skin

  • Mucous membranes

• Central Lymphoid Organs:

  • Bone marrow

  • Thymus gland

• Peripheral Organs:

  • Lymph nodes

  • Tonsils and adenoids

  • Spleen

  • Appendix

  • Peyer's patches

• Cellular Systems:

  • Inflammatory response

  • Immune cells

Essential Functions of the Immune System

• Defense:

  • Destroys or neutralizes foreign antigens.

• Homeostasis Maintenance:

  • Digests and removes damaged host cells.

  • Ensures body cell types remain uniform and unchanged.

• Surveillance:

  • Performs detection and maintains immunological memory.

  • Recognizes mutations arising within body cells as foreign and destroys them.

Classification of Immune Responses

• Innate (Natural) Immunity:

  • Serves as primary body defense.

  • Exists without prior exposure to an antigen.

  • Provides non-specific responses (such as the inflammatory response).

  • Is species-specific.

• Acquired (Adaptive) Immunity:

  • Provides specific responses tailored to individual antigens.

  • Involves the development of antibodies and immunological memory.

  • Comprises T cell-mediated immunity and B cell-mediated humoral immunity (forming antibodies following antigen exposure).

  • Natural Acquired Immunity: Achieved via direct exposure to an antigen.

  • Artificial Acquired Immunity: Achieved through immunizations or vaccines.

The Inflammatory Response: Sequential Phases

• Overview:

  • Acts as the immediate first response to tissue injury.

  • Functions as a sequential biochemical and cellular reaction.

  • Neutralizes and dilutes inflammatory agents.

  • Removes necrotic tissue materials.

  • Establishes an environment conducive to tissue healing and repair.

• Phase 1: Vascular Response:

  • Histamine and other chemical mediators are released at the injury site.

  • Vasodilation occurs, increasing localized blood flow.

  • Blood filtration pressure increases within local capillaries.

  • Endothelial retraction occurs, increasing cellular permeability.

  • Plasma proteins exert oncotic pressure, drawing fluid into the surrounding tissue to dilute foreign agents, resulting in localized swelling.

  • The plasma protein fibrinogen leaves the bloodstream and converts into fibrin.

  • Fibrin strengthens blood clots, trapping bacteria and providing a structural framework for healing.

  • Localized blood flow slows down.

• Phase 2: Cellular Response:

  • Chemotaxis signals and directs immune cells to the affected site.

  • Neutrophils arrive first at the site.

  • Neutrophils phagocytize damaged cells and microbes, subsequently dying and accumulating as pus.

  • Bone marrow releases additional cellular defenses into circulation.

  • Monocytes migrate to the tissue and transform into macrophages, acting as the primary cleanup crew.

  • Lymphocytes arrive to manage humoral and cell-mediated immune responses.

• Phase 3: Exudate Formation:

  • Fluid and white blood cells (WBCs) from circulation accumulate at the injury site.

  • The exact nature and quantity of exudate depend on the type, severity of injury, and tissue involved.

• Phase 4: Healing:

  • Consists of regeneration and repair.

  • Cellular capacity for regeneration depends on tissue type.

  • Rapid regeneration is normal for skin, bone marrow, and mucous membranes of the gastrointestinal (GI), genitourinary (GU), and reproductive tracts.

  • Regeneration of muscle and nerve tissue is significantly more problematic.

Clinical Manifestations of Inflammation

• Local Manifestations (HELPR):

  • Heat

  • Edema

  • Loss of function

  • Pain

  • Redness

• Regional Manifestations:

  • Lymph node swelling resulting from the accumulation of phagocytes, bacteria, and destroyed lymphatic tissue.

• Systemic Manifestations:

  • Fever.

  • Leukocytosis, defined as total WBC count $> 10\text{,}000\,\text{cells/mm}^3$.

  • Physiological benefits of elevated body temperature: direct elimination of microorganisms, enhancement of phagocytosis, increased proliferation of T cells, and enhancement of antiviral interferon activity.

Inflammatory Outcomes and Classification

• Inflammatory Resolution Outcomes:

  • Resolution: Normal tissue healing takes place, returning the area completely to baseline structure and function.

  • Progression: Transitions to chronic inflammation if the offending agent is not eliminated; dependent on extent of tissue damage and cellular regenerative capacity.

  • Scarring and Fibrosis: Occurs following substantial tissue damage or within tissue incapable of regeneration.

• Temporal Types of Inflammation:

  • Acute Inflammation: Healing occurs within $2 - 3\,\text{weeks}$; leaves no residual damage. Examples include insect bites, blisters, cysts, and minor burns.

  • Sub-acute Inflammation: Lasts from weeks to months; carries potential for residual tissue damage. An example includes infective endocarditis.

  • Chronic Inflammation: Lasts from weeks to years; continuous chronicity inflicts progressive tissue damage. Examples include atherosclerosis and rheumatoid arthritis.

Plasma Protein Systems in Inflammation

• Complement System:

  • Collaborates with both innate and humoral immune responses to support body defense.

  • Enhances the capacity of antibodies and phagocytic cells to clear microbes and damaged cells.

  • Promotes inflammatory signaling.

  • Directly attacks pathogen plasma membranes.

• Clotting System:

  • Stops active bleeding.

  • Traps invading bacteria.

  • Provides a structural mesh framework for tissue healing.

• Kinin System:

  • Utilizes hundreds of cytokines as chemical messengers of the immune system.

  • Induces vasodilation and stimulates cellular proliferation.

Chemical Mediators of Inflammation

• Histamine: Stored and released by mast cells; induces vasodilation, smooth-muscle constriction, tissue swelling, and localized itching.

• Leukotrienes: Stored and released by mast cells; produce physiological effects similar to histamine; significantly contribute to asthma and allergy symptoms.

• Prostaglandins: Present across most body tissues, stored and released by mast cells; increase capillary permeability, attract white blood cells to inflammatory sites, induce pain, and mediate fever.

• Bradykinin: Exists in an inactive precursor form in plasma and mast cells; acts as a potent vasodilator that causes pain; produces effects similar to histamine; degraded by angiotensin-converting enzyme (ACE).

• Complement: Comprises a cascade series of at least $20$ distinct proteins that neutralize or destroy antigens; stimulates histamine release from mast cells.

• C-Reactive Protein (CRP): A plasma-derived protein that acts as an early clinical biomarker for acute inflammation.

• Cytokines: Secreted proteins produced by macrophages, leukocytes, and dendritic cells that mediate, modulate, and regulate immune and inflammatory reactions.

The Arachidonic Acid Pathway and Cyclooxygenase Enzymes

• Pathway Initiation:

  • Tissue injury prompts the conversion of cell membrane phospholipids into arachidonic acid.

  • Arachidonic acid is metabolized via two distinct enzyme pathways: Lipoxygenase and Cyclooxygenase (COX).

• Lipoxygenase Pathway:

  • Converts arachidonic acid into leukotrienes, leading to bronchoconstriction.

• Cyclooxygenase-1 (COX-1) Pathway (Constitutional):

  • Present continuously across all physiological tissues.

  • Generates cytoprotective prostaglandins.

  • Functions: Protects gastric mucosa, reduces gastric acid secretion, promotes renal blood flow, and aids platelet aggregation.

  • Inhibition of COX-1: Leads to adverse effects including gastrointestinal bleeding, gastric upset, and reduced renal function.

  • Inhibitors: Aspirin and non-selective NSAIDs.

• Cyclooxygenase-2 (COX-2) Pathway (Inducible):

  • Formed only following tissue injury; induced by inflammatory cytokines and growth factors.

  • Generates inflammatory prostaglandins.

  • Functions: Recruits inflammatory cells, sensitizes peripheral skin pain receptors, and regulates hypothalamic temperature control (mediating central fever).

  • Inhibition of COX-2: Results in direct suppression of inflammation and pain.

  • Inhibitors: Selective COX-2 inhibitors, Aspirin, and non-selective NSAIDs.

Cellular Components of the Immune System

• Total White Blood Cell (WBC) Count: Normal range is 4\text{,}500 - 10\text{,}000\,\text{cells/mm}^3$.\n\n* **Granulocytes (Phagocytes):**\n * **Neutrophils:** Account for 55\% - 70\% of total WBCs.\n * **Bands:** Immature form of neutrophils released during high demand.\n * **Eosinophils:** Account for 4\% of total WBCs.\n * **Basophils:** Account for 1\% of total WBCs.\n\n* **Agranulocytes:**\n * **Monocytes / Macrophages:** Monocytes account for 2\% - 8\% of total circulating leukocytes/lymphocytes and function as phagocytes.\n * **Lymphocytes:** Comprise specialized B cells and T cells.\n\n\n# Monocyte and Macrophage Physiology\n\n* **Monocyte Characteristics (2\% - 8\% of total pool):**\n * Capture, process, and present antigens to lymphocytes.\n * Stimulate either humoral or cell-mediated immune responses.\n * Secrete regulating cytokines.\n\n* **Macrophage Characteristics:**\n * Represent monocytes that have migrated and fixed within specific body tissues.\n * Assigned unique anatomical names based on tissue site.\n * Function primary to capture and destroy foreign antigens.\n\n\n# Lymphocyte Subsets: T Cells and B Cells\n\n* **T Lymphocytes (T Cells):**\n * Responsible for cell-mediated immunity.\n * Coordinate both cellular and humoral immunity pathways.\n * Produced in the bone marrow but migrate and mature within the Thymus gland.\n\n* **B Lymphocytes (B Cells):**\n * Responsible for humoral immunity.\n * Produced and mature in the bone marrow.\n * Differentiate into specialized plasma cells that secrete antigen-specific antibodies (immunoglobulins).\n * **Immunoglobulin Classes:** IgG$,$IgA$,$IgM$,$IgD$,$IgE.\n\n\n# T Cell Surface Markers and Functional Subsets\n\n* **Surface Molecules:** Mature T cells display specific Clusters of Differentiation (CD) markers.\n\n* **T Helper Cells (CD4^+):**\n * Function as the master switch of the entire immune system.\n * Regulate both cell-mediated and humoral antibody responses through targeted cytokine release.\n * Stimulate the production and activity of Natural Killer (NK) cells.\n * NK cells specifically target and destroy tumor cells and virus-infected cells.\n * Differentiate into specialized cytokine-producing cells involved in allergic responses.\n\n* **T Cytotoxic Cells (CD8^+):**\n * Directly attack specific foreign antigens.\n * Provide vital control against viruses and intracellular bacteria.\n * Release cytolytic enzymes to destroy targets after activation via antigen-presenting macrophages.\n * A subset differentiates into long-lived memory T cells.\n\n\n# Acquired Immunity Mechanisms: Humoral vs. Cell-Mediated\n\n* **Cell-Mediated Immunity (T Cells):**\n * Directly activates various components of the immune system.\n * Coordinates cellular and humoral immune branches.\n * Cytotoxic T cells directly attack target antigens.\n * Response lag time is 24 - 48\,\text{hours}.\n * Memory T cells launch immediate attacks upon subsequent antigen exposures.\n\n* **Humoral Immunity (B Cells):**\n * B cells remain localized within lymphoid tissue; plasma cells release antibodies (IgG$,$IgA$,$IgM$,$IgD$,$IgE) into circulation.\n * Requires helper T cells for complete B cell activation.\n * Response lag time during first exposure is days to weeks.\n * Produces specific antibodies against identified antigens, launching immediate antibody attacks upon subsequent exposures.\n\n\n# Modes of Acquired Immunity Acquisition\n\n* **Naturally Acquired Active Immunity:** Antigens enter the host body naturally; the body independently produces specific antibodies and specialized lymphocytes.\n* **Naturally Acquired Passive Immunity:** Preformed antibodies pass naturally from mother to fetus via the placenta, or to an infant through breast milk.\n* **Artificially Acquired Active Immunity:** Antigens are introduced via vaccines; the body produces specific antibodies and specialized lymphocytes.\n* **Artificially Acquired Passive Immunity:** Preformed antibodies contained within immune serum are directly injected into the body.\n\n\n# Primary versus Secondary Immune Responses\n\n* **Primary Immune Response:**\n * Occurs immediately following initial host exposure to an antigen.\n * Measurable antibodies are formed within 14\,\text{days}$$, after which antibody titers gradually drop.

• Secondary Immune Response:

  • Occurs upon repeated host exposure to the same antigen.

  • The immune system mounts a rapid, intense, and prolonged defense.

  • Mediated by memory B cells and memory T cells established during primary exposure.

  • Provides the functional rationale for active immunization schedules and booster doses to maintain protective antibody levels.

Age-Related Physiologic Changes in Immunity

• Overall decline in immune response efficiency.

• Decline in structural external defenses (e.g., impaired mucociliary clearance).

• Increased incidence of chronic systemic diseases.

• Total WBC counts and overall numbers of lymphocytes remain stable, but cell functionality declines.

• Decreased proliferative capacity of T cells.

• Decreased response speed and efficiency of functional immune cells.

• Slowed clinical response to antibiotic therapies.

• Delayed ability to mount an adaptive response to acute immune events.

• Decreased prevalence or severity of hypersensitivity responses.

• Blunted or reduced febrile response during active infections.