Adaptive Immunity

Adaptive Immunity

Overview of Adaptive Immunity

  • Definition of Adaptive Immunity

    • A specialized immune response that develops over time and exhibits specificity for distinct pathogens.

    • Involves the lymphatic system, which plays a crucial role in the activation and function of immune cells.

Key Components of Adaptive Immunity

  • Lymphatic System

    • A network of vessels and nodes that transport lymph, a fluid containing infection-fighting white blood cells.

  • T cells

    • A type of lymphocyte responsible for cell-mediated immunity.

    • Subtypes include:

    • Helper T cells (CD4): Activate other immune cells.

    • Cytotoxic T cells (CD8): Kill virus-infected cells.

  • B cells

    • A type of lymphocyte responsible for humoral immunity.

    • Produce antibodies that neutralize pathogens.

  • Antibodies

    • Proteins produced by B cells that bind to antigens and mark them for destruction.

Course of Infection

  • Sequence of immune response events during an infection:

    • Infection establishment.

    • Induction of the adaptive immune response.

    • Memory formation.

Cells of the Immune System

  • Red Blood Cell (Erythrocyte): Provides oxygen transport but is not part of the immune system.

  • Platelets (Thrombocytes): Involved in blood clotting.

  • Hematopoietic Stem Cells: Give rise to all blood cells, including immune cells.

    • Subtypes:

    • Common Myeloid Progenitor: Leads to granulocytes (eosinophils, basophils, neutrophils) and monocytes.

    • Common Lymphoid Progenitor: Leads to lymphocytes (T cells, B cells, NK cells).

  • White Blood Cells (Leukocytes):

    • Perform various functions in the immune response such as phagocytosis and cell signaling.

Organs of the Immune System

  • Primary Lymphoid Organs

    • Thymus: Site of T cell maturation.

    • Red Bone Marrow: Site of B cell maturation.

  • Secondary Lymphoid Organs

    • Lymph Nodes: Filter lymph and activate immune responses.

    • Spleen: Filters blood and helps generate an immune response.

    • Peyer's Patches: Aggregates of lymphoid tissue in the intestine.

Principles of Adaptive Immunity

  • Only evolved in vertebrates, indicating an advantage due to increased complexity.

  • Mediated by T cells and B cells, allowing targeted immune responses to pathogens.

  • Long-lived memory T cells and B cells provide immunological memory, minimizing effects of repeat infections.

Humoral and Cellular Arms of Adaptive Immunity

  • Two main responses:

    • Humoral Response:

    • Involves B cells and production of antibodies.

    • Effective against extracellular pathogens and toxins.

    • Cellular Response:

    • Involves T cells targeting infected cells and intracellular pathogens.

  • Many pathogens require both arms for effective elimination.

Immune Activation Overview

  • Immune Activation Process:

    • Begins with activation and proliferation.

    • Followed by differentiation into effector immune cells.

  • Dendritic Cells:

    • Carry antigens from infections to lymph nodes for T cell activation.

  • Naive Cytotoxic T Cells:

    • Activated by dendritic cells to target infected cells for apoptosis.

Activation of T and B Cells During Immune Response

  • Activation of T Cells:

    • Dendritic cells migrate to lymph nodes, where they activate naive T cells.

    • Subset of activated T cells assists in B cell activation.

  • Activation of B Cells:

    • B cells bind to antigens via their receptors and receive signals from activated T cells.

    • Differentiate into antibody-secreting plasma cells or memory B cells.

Lymphocyte Circulation and Anatomy

  • Recirculation of Lymphocytes:

    • Lymphocytes move through the blood and lymphatic systems, maintaining immune surveillance.

    • Process involves the interaction of blood capillaries, lymphatic vessels, and various lymphoid tissues.

Antigen Presentation

  • T Cell Receptor (TCR) Diversity:

    • Allows T cells to recognize a vast array of antigens, primarily protein-derived molecules.

    • Antigen recognition occurs via MHC (Major Histocompatibility Complex) molecules:

    • MHC Class I: Present on almost all nucleated cells, displaying intracellular antigens (e.g., viral peptides).

    • MHC Class II: Restricted to antigen-presenting cells such as dendritic cells, macrophages, and B cells for extracellular antigens.

Antibody Structure and Function

  • Antibody Composition:

    • Composed of heavy and light chains, characterized by variable and constant regions.

    • Specificity is akin to a lock-and-key system in binding to epitope structures on antigens.

  • Functions of Antibodies:

    • Neutralization, opsonization, and complement activation leading to pathogen destruction.

    • The constant region determines the subclass of the antibody (IgM, IgG, IgA, IgE, IgD).

Immunological Memory and Vaccination

  • Memory Cells:

    • Provide faster and stronger responses upon re-exposure to the same antigen, reducing symptomatic infections.

    • More numerous than naive T and B cells specific for an antigen.

  • Vaccination Effects:

    • Enhances immune responses by stimulating memory cell production, resulting in rapid immune activation and antibody production on subsequent exposures.

Diversity Generation Mechanisms

  • VDJ (Variable-Diversity-Joining) Recombination:

    • Mechanism by which B and T cell receptors generate diversity, allowing recognition of a wide range of antigens.

    • Involves rearrangements of gene segments leading to the formation of unique receptor specificities for different antigens.

Key Concepts for Review

  • Comparison of Innate vs Adaptive Immunity

  • Roles of T cells vs B cells in the immune response

  • Differentiation between CD8 and CD4 T cells

  • Relationship between antibodies and antigens

  • Differences between MHC Class I & II in antigen presentation

  • Overview of the organs involved in the immune system and their functions.