Specific Defense System: Adaptive Immunity

Introduction to Adaptive Immunity

The adaptive (specific) immune system is a highly specialized defense mechanism that eliminates nearly any pathogen or abnormal cell in the body. Unlike innate immunity, adaptive immunity must be primed by initial exposure to a specific foreign substance, a process that takes time but results in a targeted and powerful response.

• Specificity: Recognizes and targets specific antigens.

• Systemic: Not restricted to the initial site of infection or exposure.

• Memory: After initial exposure, mounts a stronger and faster attack upon subsequent exposures to the same antigen.

Main Branches of Adaptive Immunity

• Humoral Immunity (Antibody-Mediated): Involves B lymphocytes (B cells) that produce antibodies, which circulate in body fluids and target extracellular pathogens.

• Cellular Immunity (Cell-Mediated): Involves T lymphocytes (T cells) that directly attack infected or abnormal cells, especially those harboring intracellular pathogens.

Cells of Adaptive Immunity

B Lymphocytes (B Cells)

• Produce antibodies (immunoglobulins) after activation and differentiation into plasma cells.

• Responsible for humoral immunity.

• Antibodies circulate freely in body fluids, binding to antigens and marking them for destruction.

T Lymphocytes (T Cells)

• Responsible for cellular immunity.

• Directly attack infected or abnormal cells, or help regulate other immune cells.

Major Populations of T Lymphocytes

• Helper T Cells (TH): Activate other lymphocytes (B cells, cytotoxic T cells, macrophages) and enhance the inflammatory response.

• Cytotoxic T Cells (TC): Directly kill virus-infected cells and tumor cells.

Overview: Humoral vs. Cellular Immunity

Antigens and Antigenic Determinants

Definition of Antigen

• Antigen (Ag): Any substance that provokes an immune response and mobilizes adaptive defenses. Antigens are typically large, complex molecules (often proteins or polysaccharides) with a molecular weight greater than 10,000 daltons.

• Immunogenicity: The ability of an antigen to stimulate an immune response.

• Self-antigen: Molecules normally present on the body's own cells; usually not recognized as foreign by the immune system.

Antigenic Determinants (Epitopes)

• Epitope: The specific part of an antigen that is recognized and bound by a specific antibody, B cell receptor, or T cell receptor.

• Most antigens have multiple epitopes, each capable of stimulating a distinct immune response.

• Large, complex molecules (e.g., proteins, polysaccharides) have greater antigenicity due to more epitopes.

• Small, simple molecules (e.g., plastics) have little or no antigenicity.

Example: A single bacterium may have hundreds of different epitopes, each recognized by a different antibody or lymphocyte.

Major Histocompatibility Complex (MHC) Proteins and Antigen Presentation

Role of MHC Proteins

• MHC Proteins: Cell surface proteins essential for the presentation of antigen fragments to T cells.

• Two main classes:

  • Class I MHC: Found on all nucleated cells; present endogenous (intracellular) antigens to cytotoxic T cells (CD8+).

  • Class II MHC: Found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells; present exogenous (extracellular) antigens to helper T cells (CD4+).

Antigen Presentation: The process by which antigen fragments are displayed on the cell surface by MHC proteins, allowing T cells to recognize and respond to pathogens.

Cytokines: Immune System Peptides

• Cytokines: Small proteins released by cells that affect the behavior of other cells, especially in the immune system.

• Functions include:

  • Stimulating cell surface receptors

  • Regulating intensity and duration of immune responses

  • Promoting proliferation and differentiation of immune cells

• Examples: Interferons (IFNs), Interleukins (ILs), Lymphokines

Summary Table: Key Cells and Their Functions in Adaptive Immunity

Key Terms and Definitions

• Antigen (Ag): Substance that triggers an immune response.

• Epitope: Specific region of an antigen recognized by immune cells.

• MHC (Major Histocompatibility Complex): Proteins that present antigen fragments to T cells.

• Cytokine: Signaling molecule that modulates immune responses.

• Humoral Immunity: Immunity mediated by antibodies in body fluids.

• Cellular Immunity: Immunity mediated by T cells acting directly on infected or abnormal cells.

Additional info: Adaptive immunity is essential for long-term protection against pathogens and forms the basis for immunological memory, which is the principle behind vaccination.