The Immune System: Innate and Adaptive Body Defenses

Antibodies (Immunoglobulins)

Antibodies, also known as immunoglobulins, are specialized proteins produced by plasma cells in response to antigens. They play a crucial role in the body's defense by specifically binding to antigens and facilitating their removal.

• Structure: Each antibody consists of four polypeptide chains: two identical heavy (H) chains and two identical, shorter light (L) chains. Each chain has a variable region (antigen-binding) and a constant region (class-specific).

• Classes: There are five main classes of antibodies, each with distinct roles:

  • IgM

  • IgG

  • IgA

  • IgD

  • IgE

• Functions:

  • Neutralization: Antibodies block specific sites on pathogens or toxins, neutralizing their harmful effects.

  • Agglutination: Antibodies cross-link antigens on cells, causing clumping (agglutination).

  • Precipitation: Soluble antigens are cross-linked into large complexes that precipitate out of solution.

  • Complement Activation: Antibodies bound to antigens activate the complement system, leading to cell lysis.

Example: IgG is the most abundant antibody in plasma and is the only class that crosses the placenta to provide passive immunity to the fetus.

Cellular Immunity: T Lymphocytes

Cellular immunity is mediated by T lymphocytes, which either direct adaptive immune responses or attack infected or abnormal cells directly.

• Major T Cell Types:

  • CD4 Cells: Usually become helper T cells (activate B cells, T cells, and macrophages) or regulatory T cells (moderate immune response).

  • CD8 Cells: Become cytotoxic T cells that destroy infected or abnormal cells.

• Antigen Presentation: T cells require antigens to be presented on MHC (Major Histocompatibility Complex) proteins.

  • Class I MHC: Found on all nucleated cells (except RBCs); present endogenous antigens (from within the cell).

  • Class II MHC: Found on antigen-presenting cells; present exogenous antigens (from outside the cell).

  • CD4 cells recognize antigens on class II MHC; CD8 cells recognize antigens on class I MHC.

• Activation and Clonal Selection:

  • T cell receptors must recognize both the MHC protein and the antigen (double recognition).

  • Co-stimulatory signals from antigen-presenting cells are required for full activation.

  • Activated T cells proliferate and differentiate into effector and memory cells.

  • Cytokines (e.g., interferons) are secreted to amplify the immune response.

• Effector T Cell Roles:

  • Helper T cells: Stimulate proliferation of B cells and other T cells.

  • Cytotoxic T cells: Directly attack and kill infected or cancerous cells using perforins, granzymes, or by inducing apoptosis.

  • Regulatory T cells: Suppress immune responses to prevent overactivity and autoimmunity.

Example: Cytotoxic T cells are essential for eliminating virus-infected cells and cancer cells.

Role of MHC Proteins in Cellular Immunity

MHC proteins are critical for antigen presentation and T cell activation.

Organ Transplants and Prevention of Rejection

Organ transplantation aims to replace diseased organs with healthy ones from donors. Immune rejection is a major challenge due to immune recognition of foreign tissues.

• Allografts: Transplants between individuals of the same species (most common type).

• Immune Response: Cytotoxic T cells, NK cells, and antibodies can attack transplanted tissue.

• Immunosuppressive Therapy: Drugs are used to suppress immune rejection but increase susceptibility to infections.

Example: Kidney transplants require careful tissue matching and lifelong immunosuppression.

Immune System Disorders

Abnormal immune responses can lead to immunodeficiencies, autoimmune diseases, or hypersensitivities.

• Immunodeficiencies: Conditions where immune components are deficient or function abnormally.

  • SCID (Severe Combined Immunodeficiency): Congenital; deficit of B and T cells.

  • AIDS (Acquired Immune Deficiency Syndrome): Acquired; destruction of helper T cells impairs both T and B cell function.

• Autoimmune Diseases: The immune system attacks self-tissues due to loss of self-tolerance.

  • Treatment involves suppressing the immune system (e.g., blocking cytokines or co-stimulatory factors).

  • Failure of self-tolerance may occur when foreign antigens resemble self-antigens or new self-antigens appear.

• Hypersensitivities: Overactive immune responses cause tissue damage in response to harmless substances (e.g., allergies).

Example: Allergic reactions (immediate hypersensitivity) involve rapid immune responses to allergens, mediated by IgE antibodies.

Selected Cytokines (Summary Table)

Cells and Molecules of the Adaptive Immune Response (Summary Table)

Key Terms and Definitions

• Antigen: Any substance that can provoke an immune response.

• Antibody: Protein produced by B cells that binds specifically to an antigen.

• MHC (Major Histocompatibility Complex): Cell surface proteins essential for antigen presentation to T cells.

• Cytokine: Chemical messenger that regulates immune responses.

• Self-tolerance: The immune system's ability to avoid attacking the body's own tissues.

Additional info: The immune system's complexity ensures both specificity and regulation, preventing both infection and autoimmunity.