Chapter 15

Overview of the Adaptive Immune Response

• Third Line of Defense: Unlike innate immunity, the adaptive response is highly specific and provides long-term protection.

• Primary Response: Occurs upon the first exposure to a pathogen, taking approximately $10$ to $14$ days to fully develop.

• Secondary Response: Subsequent exposures trigger a rapid, intense response due to immunologic memory.

• Mechanism of Memory: Memory B cells differentiate into plasma cells, which produce large quantities of antibodies to neutralize pathogens before illness occurs.

Humoral vs. Cell-Mediated Immunity

• Humoral Immunity: Targets extracellular pathogens and is mediated by B cells (B lymphocytes) that mature in the bone marrow.

• Cell-Mediated Immunity (CMI): Targets intracellular pathogens and is mediated by T cells (T lymphocytes) that mature in the thymus.

• Integrated System: Helper T cells ($CD4$ cells) are the central regulators of both branches; their destruction (e.g., by HIV) compromises the entire immune system.

Antigens and Antibody Structure

• Antigens: Compounds that elicit an immune response; specifically, epitopes are the antigenic determinants on a microbe's surface.

• Antibody Structure: Y-shaped proteins (immunoglobulins) consisting of four polypeptide chains.

  • Fab Region: The variable region containing the antigen-binding site.

  • Fc Region: The constant region that remains the same for a specific class of antibody.

• Protective Outcomes of Binding:

  • Neutralization: Preventing a virus or toxin from binding to host receptors.

  • Immobilization: Preventing bacterial adherence.

  • Cross-linking: Clumping multiple bacteria together for efficient phagocytosis.

  • Opsonization: Coating the pathogen to make it easier for phagocytes to recognize.

Classes of Antibodies

• $IgM$: A large pentamer; the first antibody produced during a primary response and the only one produced by a fetus in utero.

• $IgG$: The most abundant and long-lasting; it crosses the placenta, is found in breast milk, and is the primary antibody of the secondary response.

• $IgA$: A dimer found in secretions like mucus, tears, saliva, and breast milk; provides mucosal protection.

• $IgD$: Involved in the maturation of the immune response/B cells.

• $IgE$: Involved in allergic reactions, triggering histamine release from mast cells.