CH 17 IMMUNITY

The Battle Within: Chapter 17 - Adaptive Immunity: Specific Defenses of the Host

Things to Learn

  • Differentiate innate from adaptive immunity.
  • Differentiate humoral from cellular immunity.
  • Define antigen, epitope, and hapten.
  • Explain the function of antibodies and describe their structural and chemical characteristics.
  • Name one function for each of the five classes of antibodies.
  • Compare and contrast T-dependent and T-independent antigens.
  • Differentiate plasma cells from memory cells.
  • Describe clonal selection.
  • Describe how a human can produce different antibodies.
  • Describe four outcomes of an antigen–antibody reaction.
  • Describe at least one function of the following: M cells, TH cells, TC cells, Treg cells, CTLs, NK cells.
  • Differentiate T helper, T cytotoxic, and T regulatory cells.
  • Differentiate TH1, TH2, and TH17 cells.
  • Define apoptosis.
  • Define antigen-presenting cell.
  • Describe the function of natural killer cells.
  • Describe the role of antibodies and natural killer cells in antibody-dependent cell-mediated cytotoxicity.
  • Identify at least one function of each of the following: cytokines, interleukins, chemokines, interferons, TNF, and hematopoietic cytokines.

Definitions - Immunity

  • Innate Immunity: Defenses against any pathogen.
  • Adaptive Immunity: Induced resistance to a specific pathogen.
  • Major Histocompatibility Complex (MHC): Molecules expressed on mammalian cells that present antigens to T cells.
  • T-dependent Antigens: Antigens presented with (self) MHC to T helper cells that produce cytokines to activate B cells.
  • T-independent Antigens: Stimulate B cells to produce antibodies directly, without T cell assistance.

Definitions - Antigens

  • Antigen (Ag): A substance that causes the body to produce specific antibodies or sensitized T cells. Antibodies interact with epitopes, or antigenic determinants.
  • Hapten: An antigen that is combined with carrier molecules; too small to provoke an immune response independently.

The Adaptive Immune System

  • Adaptive Immunity: Defenses that target a specific pathogen; acquired through infection or vaccination.
  • Primary Response: The first time the immune system combats a particular foreign substance, which is slow to develop.
  • Secondary Response: Subsequent interactions with the same foreign substance which is faster and more effective due to immunological memory.
Example of Secondary Response:
  1. Exposure to a virus.
  2. Development of specific immune response (takes time).
  3. Virus is stopped.
  4. Subsequent attacks are responded to faster and more effectively.
  5. Results in permanent immunity.

Dual Nature of the Adaptive Immune System

  • Humoral Immunity: Produces antibodies combating foreign molecules known as antigens; mediated by B cells which mature in red bone marrow.
  • Cellular Immunity: Produces T lymphocytes; recognizes antigenic peptides processed by phagocytic cells and matures in the thymus. Secretes cytokines instead of antibodies.

Humoral vs. Cellular Immunity

  • Humoral:

    • Mediated by B lymphocytes; differentiates into plasma cells for antibody production.
    • Forms immunological memory.

  • Cellular:

    • Mediated by T cells; includes cytotoxic T cells (CTLs) which attack infected cells, and helper T cells that assist in the immune response.

Antigens: Properties and Characteristics

  • Antigens: Substances that cause the production of antibodies, often components of invading microbes.
  • Epitopes: Specific regions on an antigen that induce an immune response and are recognized by antibodies.
  • Haptens: Small antigens that cannot provoke immune responses independently without attaching to carrier molecules.
Properties of Antigens
  • Recognized as foreign by the host organism.
  • Typically macromolecules (proteins, carbohydrates).
  • Vary in ability to stimulate immune response; immune system recognizes epitopes or antigenic determinants.

Antibody Structure and Functions

  • Antibodies (or Immunoglobulins): Glycoprotein molecules composed of heavy and light chains, linked by disulfide bonds.
  • Antibody Structure:
    • H chains: two heavy chains.
    • L chains: two light chains.
    • Fc region: Constant region that binds to Fc receptors on cells.
    • Fab region: The portion that binds with antigens.
Functions of Antibodies:
  1. Neutralization of toxins or viruses.
  2. Immobilization of flagella; preventing adherence.
  3. Agglutination and precipitation of pathogens.
  4. Opsonization: Enhancing phagocytosis by coating antigens with antibodies.
  5. Activation of Complement: Initiating a cascading series of reactions that enhance pathogen clearance.
  6. Antibody-Dependent Cellular Cytotoxicity (ADCC): NK cells destroy target cells marked by antibodies.
Classes of Antibodies

  • IgG:

    • Monomer, makes up 80-85% of total serum Ig, crosses the placenta, involved in opsonization and neutralization.
    • Half-life = 23 days.

  • IgM:

    • Pentamer made up of 5-10% of serum antibodies, first produced in response to antigens.
    • Most effective in agglutination and complement fixation.
    • Half-life = 5 days.

  • IgA:

    • Dimer in secretions, present in mucous membranes, inhibits adherence of microorganisms; half-life = 6 days.

  • IgD:

    • Monomer, found on B cells, initiates immune responses; half-life = 3 days.

  • IgE:

    • Monomer, involved in allergic responses; binds to basophils and mast cells; half-life = 2 days.

Major Histocompatibility Complex (MHC)

  • MHC: Cluster of genes coding for glycoprotein molecules involved in cell-to-cell recognition.
  • MHC Class I: Presents endogenous antigens (e.g., virus-infected cells).
  • MHC Class II: Presents exogenous antigens (e.g., bacteria).
  • Essential in organ/tissue transplantation and immune system recognition functions.

T Cells and Cellular Immunity

  • T Cells: Respond to antigens via T-cell receptors (TCRs). Require antigen-presenting cells (APCs).
  • T Helper Cells: Recognize and bind to antigens presented on MHC II molecules, produce cytokines that regulate immune responses.
  • Cytotoxic T Cells (CTL): Target and kill infected or cancerous cells by inducing apoptosis.
Types of T Helper Cells
  • TH1 cells: Activate macrophages, support cell-mediated immunity.
  • TH2 cells: Assist B cells in antibody production, relevant in allergy and asthma responses.
  • TH17 cells: Play a role in autoimmune conditions, stimulate the innate immune response.

Clonal Selection and Expansion

  • Process by which a lymphocyte recognizes its specific antigen, binds to it, and undergoes proliferation to form a clone that can effectively combat the antigen.
  • Plasma Cells: Activated B cells that secrete large amounts of antibodies.
  • Memory Cells: Provide lasting immunity, can quickly produce antibodies upon re-exposure to the antigens.

Adaptive Immunity

  • Naturally Acquired Active Immunity: Resulting from natural infection.
  • Naturally Acquired Passive Immunity: Antibodies transferred from mother to infant via placenta or breast milk.
  • Artificially Acquired Active Immunity: Injection of vaccine containing antigens to stimulate an immune response.
  • Artificially Acquired Passive Immunity: Injection of preformed antibodies from an immune individual.
Summary of Immune Responses
  • Primary Immune Response: Takes time to generate antibodies upon first exposure to an antigen; initiation involves activation of B cells primarily producing IgM, then class switching to IgG as memory cells are formed.
  • Secondary Immune Response: Faster due to the activation of memory cell responses amplifying antibody production upon re-exposure to the antigen.