Adaptive Immunity: Study Guide and Key Concepts

Adaptive Immunity

Overview of Adaptive Immunity

The adaptive immune system is the body's third line of defense, providing specific and long-lasting protection against pathogens. Unlike innate immunity, adaptive responses are slower to develop but generate immunological memory, allowing for a more rapid and robust response upon subsequent exposures to the same antigen.

• Specificity: Targets particular pathogens using unique molecular markers (antigens).

• Slower Onset: Primary response is slow; secondary response is faster and stronger due to memory cells.

• Immunological Memory: Enables long-term protection.

Branches of Adaptive Immunity

• Cellular Immunity: Mediated by T cells, which directly attack infected or abnormal cells.

• Humoral Immunity: Mediated by B cells, which produce antibodies that neutralize pathogens.

• Both branches work together to eliminate antigens and generate memory cells for future protection.

Four Stages of the Adaptive Immune Response

1. Antigen Presentation: Antigen-presenting cells (APCs) display antigens to lymphocytes.

2. Lymphocyte Activation: T and B cells are activated upon recognizing their specific antigen.

3. Proliferation and Differentiation: Activated lymphocytes multiply and differentiate into effector and memory cells.

4. Antigen Elimination and Memory: Effector cells eliminate the antigen; memory cells persist for rapid future responses.

Lymphocytes

T Cells

• Mature in the thymus.

• Types:

  • CD4+ Helper T cells: Coordinate immune responses; subtypes include:

    • TH1: Promote cellular immunity (activate macrophages, cytotoxic T cells).

    • TH2: Promote humoral immunity (stimulate B cells).

    • Treg (Regulatory T cells): Suppress immune responses to maintain self-tolerance.

  • CD8+ Cytotoxic T cells: Destroy infected or abnormal cells using perforins and granzymes.

B Cells

• Mature in the bone marrow.

• Differentiate into plasma cells that secrete antibodies.

Antigens and Immunogenicity

• Antigen: Any substance that triggers an immune response.

• Immunogenicity: The ability of an antigen to provoke an immune response. Proteins are the most immunogenic, followed by polysaccharides, then lipids.

• Epitopes: Specific regions of an antigen recognized by immune cells.

• Haptens: Small molecules that are only immunogenic when attached to a carrier protein.

Receptors and Specificity

• T Cell Receptor (TCR): Recognizes antigen fragments presented with MHC molecules.

• B Cell Receptor (BCR): Binds directly to free antigens.

• Each lymphocyte is specific for a single epitope.

Clonal Expansion

Upon activation, lymphocytes undergo clonal expansion, producing many identical cells:

• Effector cells: Actively participate in eliminating the antigen.

• Memory cells: Remain in the body for rapid response upon re-exposure.

Major Histocompatibility Complex (MHC)

• MHC I: Found on all nucleated cells; presents intracellular antigens to CD8+ cytotoxic T cells.

• MHC II: Found on antigen-presenting cells; presents extracellular antigens to CD4+ helper T cells.

T Cell Activation

• Requires two signals:

  1. Recognition of antigen-MHC complex by TCR.

  2. Co-stimulatory signal from the APC.

B Cell Activation

• T-dependent activation: Requires help from CD4+ helper T cells; results in strong memory formation.

• T-independent activation: Direct activation by certain antigens; produces a weaker memory response.

Antibody Functions

• Neutralization: Blocks pathogen binding to host cells.

• Opsonization: Coats pathogens to enhance phagocytosis.

• Agglutination/Precipitation: Clumps antigens for easier removal.

• Complement Activation: Triggers the complement cascade to destroy pathogens.

Antibody Structure

• Composed of 2 heavy chains and 2 light chains.

• Variable region: Binds to specific antigen epitopes.

Antibody Isotypes

Isotype	Main Features
IgG	Most abundant in blood; provides long-term immunity and memory
IgA	Found in mucosal areas (e.g., saliva, tears, breast milk)
IgM	First antibody produced in primary response
IgE	Involved in allergic reactions and defense against parasites
IgD	Functions mainly as a B cell receptor

Primary vs. Secondary Immune Response

• Primary Response: Slow onset; IgM is produced first.

• Secondary Response: Rapid and robust; high levels of IgG are produced due to memory cells.

Self-Tolerance

• Prevents the immune system from attacking the body's own tissues (autoimmunity).

• T cells are tested for self-reactivity in the thymus.

• B cells are tested in the bone marrow.

Types of Immunity

Type	How Acquired	Example
Natural Active	Exposure to pathogen	Infection
Artificial Active	Immunization	Vaccination
Natural Passive	Transfer of antibodies	Maternal antibodies (placenta, breast milk)
Artificial Passive	Injection of antibodies	Antiserum