The Adaptive Immune Response

Chapter 15: The Adaptive Immune Response

Introduction

  • Learning changes everything.

  • Adaptive Immunity: The body's ability to recognize and defend against distinct invaders and their products.

Types of Immunity

  • Innate Immunity:

    • Characteristics: Born with this immunity.

    • First and second lines of defense include barriers (skin, mucous membranes) and secretions.

  • Adaptive Immunity:

    • Acquired immunity that develops as the body encounters antigens over time.

    • Relates to lymphocytes and their specific responses to pathogens.

Comparison of Innate and Adaptive Immunity

  • Innate Immunity

    • Distribution: Present in almost all multicellular eukaryotes.

    • Target: Limited number of key structures in many microbes (PAMPs).

    • Immune receptors: Pattern recognition receptors like Toll-like receptors (TLRs).

  • Adaptive Immunity

    • Distribution: Present only in vertebrates.

    • Target: Antigens, predominantly proteins (billions of different antigens).

    • Immune receptors: T cell receptors (TCRs) and antibodies.

Overview of Adaptive Immunity

  • Five key attributes:

    • Specificity: Ability to recognize specific antigens.

    • Inducibility: Can be triggered by exposure to antigens.

    • Clonality: Proliferation of specific lymphocytes upon antigen exposure.

    • Unresponsiveness to self: Typically does not respond to the body’s own cells.

    • Memory: Provides long-term protection; faster response upon re-exposure.

Lymphocytes in Adaptive Immunity

  • Types of lymphocytes:

    • B Lymphocytes (B cells): Mature in bone marrow.

    • T Lymphocytes (T cells): Mature in the thymus.

  • Types of adaptive immune responses:

    • Cell-mediated immune responses: Involve T cells (e.g., defense against intracellular pathogens).

    • Humoral immune responses: Involve B cells and antibodies.

Hematopoiesis

  • Involves stem cells in the bone marrow and different lineages of leukocytes:

    • Myeloid stem cells lead to various blood cells including erythrocytes and leukocytes.

    • Lymphoid stem cells produce lymphocytes (B and T cells).

Characteristics and Functions of Antigens

  • Antigens: Recognized as foreign by the immune system, leading to an immune response.

    • Epitopes: Three-dimensional regions of antigens recognized by immune cells.

    • Large foreign molecules provoke the strongest responses (proteins, polysaccharides).

    • Categories of antigens:

    • Exogenous antigens: From outside the body.

    • Endogenous antigens: Produced by infected cells.

    • Autoantigens: Components of normal body cells, can lead to autoimmune responses.

T Lymphocytes (T Cells)

  • Function: T cells act primarily against cells harboring intracellular pathogens.

  • T cell receptors (TCRs): Bind only to epitopes presented by MHC molecules.

  • Major Histocompatibility Complex (MHC): Important in antigen presentation; types are:

    • MHC class I: Present on all nucleated cells, recognized by CD8 T cells.

    • MHC class II: Present on antigen-presenting cells (APCs), recognized by CD4 T cells.

Activation and Function of T Cells

  • T cell Activation Steps:

    1. Antigen presentation by APCs.

    2. Helper T cell differentiation and clonal expansion.

    3. Memory cell formation.

  • Types of T cells:

    • Cytotoxic T cells (Tc): Kill infected or cancerous cells.

    • Helper T cells (Th): Assist other immune cells.

    • Regulatory T cells: Repress immune responses to maintain tolerance.

B Lymphocytes (B Cells) and Antibodies

  • B cells: Found in spleen and lymph nodes; responsible for producing antibodies.

  • B cell receptor (BCR): Recognizes antigens; binds to specific epitopes.

  • Activated B cells differentiate into plasma cells and memory B cells.

Antibody Structure and Function

  • Structure: Y-shaped protein, consists of heavy and light chains with variable and constant regions.

  • Functions of antibodies include:

    • Neutralization: Bind to and block toxins or pathogens.

    • Opsonization: Mark pathogens for ingestion by phagocytes.

    • Complement Activation: Trigger inflammation and lysis of pathogens.

    • Agglutination: Clumping of pathogens for easier clearance.

Classes of Antibodies

  • IgM: First antibody produced, pentameric structure, effective in complement activation.

  • IgG: Most abundant, long-lasting, crosses placenta to protect fetus.

  • IgA: Found in secretions (saliva, tears), important for mucosal immunity.

  • IgE: Involved in allergic responses and defense against parasites.

  • IgD: Function not clearly defined, but associated with B cell maturation.

Immune Responses

  • Primary Response: First response to an antigen, takes time to develop;

  • Secondary Response: Faster, due to memory B cells; more effective with greater antibody production.

  • T-independent vs T-dependent Immunity: T-independent antigens activate B cells directly, whereas T-dependent antigens require Th cells for B cell activation.

Summary of Adaptive Immune Response

  • Adaptive immunity is characterized by specificity, memory, and the ability to distinguish self from non-self. Involves both humoral and cell-mediated responses, utilizing a diverse array of lymphocytes and their receptors to effectively neutralize threats to the body.