Adaptive Immune System Summary

Overview

  • The adaptive immune system recognizes and neutralizes specific foreign substances.

  • It amplifies inflammation and activates complement.

  • Key features: antigen-specificity, systemic response, and memory.

  • Two arms: Humoral (B cell) and Cellular (T cell) immunity.

Antigens (AG)

  • Substances that mobilize the immune system; targets are typically non-self molecules.

  • Examples: viruses, bacteria, toxins, cancer cells, pollen.

Complete Antigens

  • Cause immunogenicity and reactivity; include foreign proteins, nucleic acids, some lipids, and large polysaccharides.

  • Trigger the full immune response.

Haptens (Incomplete Antigens)

  • Small molecules that become immunogenic when attached to protein carriers.

  • Found in drugs, poison ivy, detergents, and cosmetics.

Antigenic Determinants

  • Specific parts of an antigen where antibodies and activated lymphocytes bind.

Self-Antigens: MHC Proteins

  • Protein molecules on our cells (not antigenic to us but can be to others).

  • MHC proteins identify cells as self.

  • Two classes:

    • Class I MHC: on virtually all body cells.

    • Class II MHC: only on Antigen Presenting Cells (APCs).

Function of MHC Proteins

  • MHC molecules display a peptide fragment.

  • On infected cells, Class I MHC proteins bind to foreign antigen fragments, signaling "non-self."

Class I MHC

  • Found on all human cells.

  • Displays self-antigens normally, non-self antigens if infected, activating immune cells.

Class II MHC

  • Found only on APCs.

  • Present self and foreign proteins.

Cells of the Adaptive Immune System

  • B lymphocytes: humoral immunity.

  • T lymphocytes: cell-mediated immunity.

  • Antigen-presenting cells (APCs): engulf antigens and present them to T cells.

Lymphocyte Maturation

  • B cells mature in bone marrow; T cells in the thymus.

  • Must develop:

    • Immunocompetence: recognize one specific antigen.

    • Self-tolerance: be unresponsive to self-antigens.

T Cell Selection

  • Occurs in the thymus.

  • Positive selection: recognize self-MHC.

  • Negative selection: not recognize self-antigens.

B Cell Selection

  • Occurs in bone marrow; B cells become immunocompetent and self-tolerant.

  • Self-reactive B cells are inactivated or killed.

Immunocompetent Cells

  • Display a unique receptor for a distinct antigen.

  • Genes, not antigens, determine recognition.

Antigen-Presenting Cells (APCs)

  • Engulf foreign particles and present antigen fragments on MHC II to T cells.

  • Major APCs: dendritic cells, macrophages, B cells.

  • Dendritic cells initiate adaptive immunity, migrating to lymph nodes.

Humoral Immunity: B Cells Response

  • Antigen challenge: first encounter between antigen and naive B cell.

  • Provokes a humoral immune response.

Clonal Selection

  • Antigen binding activates B cell, leading to receptor-mediated endocytosis.

  • Stimulated B cell growth forms clones bearing the same antigen-specific receptors.

Clones Fate

  • Most clone cells become antibody-secreting plasma cells.

  • Some clones become memory cells for future exposures.

Immunological Memory

  • Primary response: 3-6 day lag, peak antibody levels in 10 days.

  • Secondary response: sensitized memory cells respond within hours, antibody levels peak in 2-3 days.

Active vs. Passive Immunity
Active Immunity

  • B cells produce antibodies after encountering antigens.

  • Naturally acquired: infection.

  • Artificially acquired: vaccination.

  • Vaccines provide immunogenic determinants without disease.

Passive Immunity

  • Antibodies acquired from external source; no immunological memory.

  • Naturally acquired: mother to fetus (IgG).

  • Artificially acquired: injection of serum with antibodies.

Antibodies (Immunoglobulins)

  • Soluble proteins secreted by activated B cells and plasma cells; bind specifically with antigens.

  • Five classes: IgM, IgA, IgD, IgG, IgE (MADGE).

Antibody Classes

  • IgM: primary response.

  • IgA: prevents pathogen attachment.

  • IgD: B cell activation.

  • IgG: most abundant, crosses placenta.

  • IgE: histamine release (allergies).

Basic Antibody Structure

  • Four polypeptide chains (two heavy, two light) linked by disulfide bonds.

  • Variable regions form antigen-binding site; constant regions determine class and function.

Antibody Diversity

  • Somatic recombination: gene segments are shuffled.

Antibody Targets

  • Antibodies inactivate and tag antigens for destruction, forming an immune complex.

  • Mechanisms: Neutralization, Agglutination, Precipitation, Complement Fixation.

Antibody Action Mechanisms

  1. Neutralization: block binding sites.

  2. Agglutination: cross-link cell-bound antigens.

  3. Precipitation: cross-link soluble molecules.

  4. Complement Fixation: activate complement, leading to cell lysis.

Monoclonal Antibodies

  • Pure antibody preparations for a single antigenic determinant.

  • Used in research and treatment of cancers.

Cell-Mediated Immune Response: T Cells

  • Needed because antibodies cannot act against intracellular antigens.

  • Mediated by T cells: CD4 (helper T cells), CD8 (cytotoxic T cells).

T Cell Function

  • Recognize processed antigen fragments on body cells via MHC.

  • Targets: infected, cancerous, and foreign cells.

Antigen Recognition and MHC Restriction

  • T cells recognize self (MHC protein) and non-self (antigen).

  • Class I MHC: CD8 T cells (endogenous antigens).

  • Class II MHC: CD4 T cells (exogenous antigens).

Class I MHC Proteins

  • Display endogenous antigens.

Class II MHC Proteins

  • Only on APCs; bind to CD4 T cells.

T-Cell Antigen Recognition

  • MHC I display activates CD8 cells to become cytotoxic T-cells.

T Cell Activation: Two-Step Process

  1. Antigen Binding: TCRs bind to antigen-MHC complex.

    • CD4 cell binds to APC with Class II MHC.

    • CD8 cell binds to APC with Class I MHC.

  2. Co-stimulation: required for T cell activation.

Role of Helper T Cells (TH) or Regulatory

  • CD4 cells become TH1 & TH2:

    • Stimulate proliferation of other T cells (TH1 activates CD8 T cell).

    • Stimulate B cells that have already become bound to antigen (TH2 activates B cells).

    • Without TH, there is no adaptive immune response.

Role of Cytotoxic T Cells (TC)

  • Activated CD8 T cells directly attack and kill other cells (infected, cancerous, foreign).

Mechanisms of Tc Action

  • Release perforin, secrete lymphotoxin, secrete gamma interferon.

Suppressor T Cells (TS)

  • Regulatory cells that suppress T and B cell activity; turn off immune system.

Cytokines

  • Mediators involved in cellular immunity; hormone-like glycoproteins released by T cells and macrophages.