Integrative Physiology: The Immune System

Immunology Study Guide

Components of the Immune System

  • Innate Immunity

  • Adaptive Immunity

Cells of the Immune System

  • White Blood Cells (Leukocytes)

    • Neutrophils

    • Most rapidly respond to tissue damage caused by bacteria.

    • Type of Phagocyte.

    • Eosinophils

    • Destroy parasites.

    • Also contribute to allergic responses.

    • Basophils

    • Release various chemicals that promote inflammation and allergic responses.

      • Chemicals include:

      • Histamine

      • Leukotrienes

      • Prostaglandins

    • Monocytes

    • Differentiate into Macrophages, which can be wandering or fixed.

    • Lymphocytes

    • B Lymphocytes

      • Responsible for antibody production (turn into plasma cells).

      • Form Memory Cells.

    • T Lymphocytes

      • Include Helper T Cells and Cytotoxic T Cells.

    • Natural Killer Cells

  • Other Immune Cells

    • Mast Cells

    • Found in connective tissue of skin and lining of respiratory and gastrointestinal tracts.

    • Dendritic Cells

    • Known also as Langerhans cells or veiled cells, found in skin and other organs.

Key Cells of the Immune System

  • Subtypes and Nicknames

    • Termed “polys” or “segs.”

    • Immature forms referred to as “bands” or “stabs.”

  • Mononuclear Phagocyte System.

  • Functions

    • Release chemicals mediating inflammation and allergic responses.

    • Ingest and destroy pathogens.

    • Destroy antibody-coated invaders.

    • Present antigens to activate immune responses.

  • Composition in Blood:

    • Basophils - Rare

    • Neutrophils - 50–70%

    • Eosinophils - 1–3%

    • Macrophages - 1–6%

    • Lymphocytes - 20–35%

  • Classifications:

    • Phagocytes

    • Granulocytes

    • Cytotoxic Cells

    • Antigen-Presenting Cells

Lymphoid Organs

  • Definition: Structures where lymphocytes develop, reside, or conduct immune responses.

  • Categories:

    • Primary Lymphoid Organs: Stem cell division and development into B and T cells.

    • Secondary Lymphoid Organs: Majority of immune responses occur here.

Primary Lymphoid Organs

  • Bone Marrow

    • Location of pluripotent stem cells, producing mature B cells and immature T cells.

  • Thymus

    • Located above the heart, contains T cells and other cells (dendritic cells, epithelial cells, macrophages).

    • Site of T cell maturation; atrophies with maturity.

Secondary Lymphoid Organs

  • Lymph Nodes

    • Scattered throughout lymphatic vessels; filter microbes, macrophages, and lymphocytes destroy filtered microbes.

  • Spleen

    • Largest lymphoid organ; removes microbes and aged/defective erythrocytes.

  • Lymphoid Nodules

    • Examples include tonsils, Peyer’s patches, appendix; lack a capsule, thus classified as non-organs.

Innate Immunity

  • Definition: Non-specific and does not have a memory component for invading pathogens.

  • Defense Mechanisms:

    • First Line of Defense: External physical and chemical barriers (skin, mucous membranes, etc.)

    • Second Line of Defense: Internal responses involving natural killer cells, phagocytes, fever, and inflammation.

First Line of Defense

  • Physical Barriers:

    • Skin (composed of epidermis and dermis, among other components).

    • Mucus, Hair, Cilia.

    • Sebum and Lysozyme in various secretions.

    • Gastric Juice (pH 1.2-3.0) and Vaginal Secretions (slightly acidic).

Second Line of Defense

  • Components:

    • Natural Killer Cells

    • Phagocytes

    • Steps include:

      1. Adherence

      2. Ingestion

      3. Digestion

      4. Killing

    • Fever and Inflammation

  • Antimicrobial Substances:

    • Interferons, complement proteins, iron-binding proteins, and antimicrobial proteins.

Phagocytosis Process

  1. Adherence: Phagocyte binds to microbe.

  2. Ingestion: Microbe ingested into a phagosome.

  3. Digestion: Fusion of phagosome with lysosome, leading to the destruction of the microbe.

  4. Killing: Remaining material forms residual body.

Inflammation

  • Definition: Non-specific response to tissue damage.

  • Cardinal Signs and Symptoms:

    • Redness (Rubor)

    • Pain (Dolor)

    • Heat (Calor)

    • Swelling (Tumor)

    • Loss of Function (Functio Laesa)

  • Stages:

    1. Vasodilation: Increased blood flow and permeability; influenced by histamines, kinins, prostaglandins, leukotrienes, and complement.

    2. Emigration of Phagocytes: Phagocytes move across capillaries through diapedesis, following chemotactic signals.

    3. Tissue Repair: After clearance of microbes, tissue repair occurs, which may lead to the formation of scar tissue.

Summary of Innate Defenses

First Line of Defense

  • Prevents microbial entry into deeper tissues via:

    • Physical Barriers: Skin, mucus, hairs, cilia

    • Chemical Barriers: Sebum, lysozyme, gastric juice, and vaginal secretions.

Second Line of Defense

  • Internal defenses include:

    • Antimicrobial substances (like interferons and complement)

    • Natural killer cells and phagocytes that eliminate pathogens and initiate tissue repair.

Adaptive Immunity

  • Definition: The capacity to respond to specific microbes, involving antigens recognized as foreign by the immune system.

  • Key Characteristics:

    • Specificity for particular foreign molecules.

    • Memory for previously encountered antigens.

Key Components

  • Lymphocytes:

    • B Cells

    • Develop into plasma cells (antibody production) and memory cells.

    • T Cells

    • Cytotoxic T Cells: CD8 positive, attack infected body cells.

    • Helper T Cells: CD4 positive, assist other cells in the immune response.

Mechanisms of Adaptive Immunity

  • Cell-Mediated Immunity: Involves T cells targeting infected or cancerous cells.

  • Antibody-Mediated Immunity (Humoral Immunity): B cells produce antibodies that neutralize pathogens in body fluids.

Antigen Functions

  • Characteristics: Antigens stimulate immune response through immunogenicity and reactivity, defined as:

    • Immunogenicity: Ability to provoke immune response by stimulating antibody production.

    • Reactivity: Antibodies bind to specific antigens that provoked them.

  • Types: Antigens can be whole microbes or parts; non-microbial antigens include pollen and transplanted tissues.

  • Epitopes: Specific sites on antigens recognized by antibodies, determining the specificity of immune responses.

Major Histocompatibility Complex (MHC)

  • Self Antigens: Body does not produce antibodies against its own MHCs, which are responsible for presenting antigens to immune cells and are unique to each individual.

  • MHC Classes:

    • Class I: Found on all nucleated cells, plays role in presenting endogenous antigens.

    • Class II: Found on Antigen-Presenting Cells (APCs), presents exogenous antigens.

T Cell Activation

  • T cells activate only when bound to a foreign antigen through MHC interaction.

  • Helper T Cell Activation:

    • Involves antigen recognition, co-stimulation, and clonal selection leading to memory and active helper T cells.

  • Cytotoxic T Cell Activation:

    • Also requires antigen recognition and co-stimulation, leading to the formation of active and memory cytotoxic T cells.

B Cell Activation and Cloning

  • B cells become activated and undergo clonal selection to form plasma cells that produce antibodies and memory B cells for future responses.

Antibody Structure and Functions

  • Structure: Antibodies consist of three regions — hinge, fab (antigen-binding), and fc (constant region); five primary classes:

    • IgG: Most common, involved in secondary responses (80%).

    • IgA: Secretory form (10-15%).

    • IgE: Associated with allergies (0.1%).

    • IgM: First response antibodies (5-10%).

    • IgD: Function unclear, found on B cells.

Antibody Functions

  • Mechanisms of Action: Antibodies neutralize, agglutinate, precipitate antigens, activate complement, and opsonize pathogens for phagocytosis.

Immunity Acquisition

  • Active Immunity: Results from the immune system's response to pathogens.

    • Natural Active Immunity: Acquired through disease exposure.

    • Artificial Active Immunity: Acquired through vaccination.

  • Passive Immunity: Receipt of antibodies from another organism, typically temporary due to lack of memory cell involvement.

    • Natural Passive Immunity: Antibodies passed from mother to child via placenta or breast milk.

    • *Artificial Passive Immunity: Serum containing antibodies from another individual.

Self-Recognition and Autoimmunity

  • The immune system must distinguish between self and non-self to prevent autoimmunity, an aberrant response against the body's own cells. Methods include:

    • Self-recognition and self-tolerance: Loss of self-tolerance leads to autoimmune disorders.

Allergic Reactions

  • Definition: Overreactions to substances tolerated by many.

  • Types:

    • Immediate Hypersensitivity: Fast, IgE mediated.

    • Delayed Hypersensitivity: Slower onset, appearing 12-72 hours post-exposure.

Neuro-Endocrine-Immune Interaction

  • Concept: The interaction and influence of the nervous, endocrine, and immune systems through shared signaling molecules and receptors, highlighting the importance of hormonal regulation in immune responses.

Contributions of the Immune System to Homeostasis

  • Integumentary System: Skin acts as a first line of defense; Langerhans cells alert immune responses.

  • Respiratory System: Lymph tissues in tonsils and alveolar macrophages protect against inhaled pathogens.

  • Nervous System: Microglia protect the brain; IL-1 from macrophages can induce a fever.

  • Endocrine System: Thymus hormones support T cell function.

  • Cardiovascular System: Spleen macrophages remove pathogens from blood.

  • Digestive System: Lymphoid tissue protects against ingested pathogens.

  • Reproductive System: Lymphoid tissues protect against sexually transmitted pathogens with maternal antibodies providing protection to infants.