Adaptive Immunity Review

Adaptive Immunity

Overview

  • Purpose:

    • Destruction of infectious microorganisms that are resistant to inflammation.

    • Long-term, highly effective protection against future exposure to the same microorganism.

  • Characteristics:

    • Inducible

    • Specific

    • Long-lived

    • Has memory

  • Elements:

    • Antigens (from the pathogen)

    • Lymphocytes (T cells, B cells)

Learning Objectives

  1. Distinguish between natural and acquired immunity.

  2. Define and describe humoral and cell-mediated immunity.

  3. Describe the differences between active and passive immunity.

  4. Define antigen and differentiate between the various types of antigens.

  5. Describe the steps of antigen binding, presenting, and processing.

  6. Define antibody, describing the molecular structure and function of an antibody.

  7. Identify the classes of immunoglobulins.

  8. Describe what is meant by direct and indirect effects of an antibody.

  9. Define and describe the differences between secretory and systemic immune systems.

  10. Discuss the roles of the various types of T lymphocytes and their development from stem cells.

  11. Define and describe clonal diversity and clonal selection.

  12. Differentiate between a primary and secondary immune response.

  13. Discuss the alterations in immunity for infants and the elderly.

Components of Adaptive Immunity

Humoral Immunity

  • Definition: Immunoglobulins (antibodies) produced by B cell lymphocytes.

  • Function: Bind to antigens on bacteria and viruses.

Cellular Immunity

  • Definition: Involves T cells (lymphocytes).

  • Functions:

    • Kill target cells directly.

    • Stimulate other immune responses.

Lymphocyte Development

B-cell Development

  • Location: Bone marrow

  • Processes:

    • Production, proliferation, and differentiation.

    • Migrate to lymphoid tissue as immunocompetent cells.

    • Each cell responds to only one specific antigen.

T-cell Development

  • Location: Thymus

  • Processes:

    • Production of antigen-specific T-cell receptors (TCRs).

    • Migrate to lymphoid tissue and mature into immunocompetent cells.

Immunity Types

Active Immunity

  • Natural Active Immunity:

    • Resulting from infection.

  • Artificial Active Immunity:

    • Resulting from vaccination.

Passive Immunity

  • Natural Passive Immunity:

    • Via maternal antibodies (e.g., breast milk).

  • Artificial Passive Immunity:

    • Use of monoclonal antibodies.

Antigens and Immunogens

Antigens

  • Function: Bind with antibodies and receptors on T and B cells.

  • Not necessarily immunogens.

Immunogens

  • Definition: Substances that induce the production of antibodies, T cells, and B cells.

  • Note: All immunogens are antigens, but not all antigens are immunogens.

Haptens

  • Definition: Too small to be immunogens by themselves, but become immunogenic upon combining with larger carrier molecules.

Immunoglobulins (Antibodies)

Classes of Immunoglobulins

  • Immunoglobulin G (IgG):

    • Most abundant (80-85%)

    • Key protective activity against infections.

    • Crosses the placenta.

  • Immunoglobulin A (IgA):

    • Two classes:

    • IgA in blood.

    • Secretory IgA (IgA-2) primarily in bodily secretions.

    • Structure: Dimer anchored by a J chain and a secretory piece for protection.

  • Immunoglobulin M (IgM):

    • Largest immunoglobulin (pentamer stabilized by a J chain).

    • First antibody produced during primary response.

    • Synthesized early in neonatal life.

  • Immunoglobulin D (IgD):

    • Low blood concentration.

    • Functions as a B-cell antigen receptor.

  • Immunoglobulin E (IgE):

    • Mediator of allergic responses and defense against parasites.

    • Initiates inflammatory reactions and binds to mast cells.

Antibody Functions

Direct Effects

  • Neutralization: Blocking pathogen entry.

  • Agglutination: Clumping pathogens.

  • Precipitation: Forming insoluble complexes.

Indirect Effects

  • Inflammation: Recruitment of immune cells.

  • Phagocytosis: Engulfment of pathogens.

  • Complement Activation: Enhancing pathogen destruction.

Clonal Diversity and Selection

Clonal Diversity

  • Location: Takes place in lymphoid organs (thymus and bone marrow).

  • Results: Production of naïve but immunocompetent T and B cells.

  • Primarily occurs in the fetus.

Clonal Selection

  • Involves:

    • Antigen processing and presentation.

    • Complex cellular interactions resulting in specific immune responses.

Antigen Processing and Presentation

  • Initiated by the interaction of T and B cells with an antigen.

  • Must be processed and presented by antigen-presenting cells (APCs).

  • Outcomes:

    • Differentiation of B cells into plasma cells (active antibody producers).

    • Differentiation of T cells into effector cells (e.g., T-cytotoxic cells).

Immune Responses

Primary Immune Response

  • Characteristics:

    • Initial exposure to the antigen.

    • Latent period or lag phase while B-cell differentiation occurs.

    • Detection of IgM antibody around days 5 to 7 post-exposure.

    • An IgG response follows.

Secondary Immune Response

  • Characteristics:

    • Rapid response upon subsequent exposure.

    • Larger antibody amounts produced due to memory cells.

    • IgG produced in significantly greater quantities may follow a transient IgM response.

T-Cell Activation

  • Mechanism: T-cell receptors (TCRs) bind to the antigen presented by APCs.

  • Functions:

    • Directly kill foreign cells via cytotoxic T cells.

    • Activate other immune cells (cytokine release).

  • Cytokine Storm: A severe immune response, noted in conditions like COVID-19.

T-Helper Cells

  • Function: Aid in driving maturation of B and T cells based on antigens.

  • Roles:

    • Facilitate interactions between APCs and lymphocytes.

    • Different subsets (e.g., Th1 for cell-mediated, Th2 for humoral immunity) based on cytokine production.

Neonatal Immunity

  • Immune characteristics of neonates:

    • Sufficient levels of IgM but deficient levels of IgG and IgA, resulting in immunologically immature responses.

  • Maternal antibodies provide initial protection during fetal circulation and the first months of life.

Aging and Immune Function

Changes in Adaptive Immunity

  • B cells, CD4 T cells, CD8 T cells, and memory T cells generally decrease in function with age.

Changes in Innate Immunity

  • Decreased function across immune cells such as neutrophils and macrophages.

  • Thymus involution leads to decreased T cell production.

  • Changes in cytokine profiles (e.g., increased IL-6, IL-12, TNF-alpha) and impaired phagocytosis.

Summary of Effects

  • Overall immune function diminishes, affecting both adaptive and innate immune responses, leading to increased susceptibility to infections and decreased efficacy of vaccines in older adults.