Adaptive Immunity: Study Guide and Key Concepts

Adaptive Immunity

Overview of Adaptive Immunity

The adaptive immune system is the third line of defense in the human body, providing a specific and targeted response to 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 specific antigens using specialized lymphocytes.

• Memory: Generates long-lasting memory cells for faster secondary responses.

• Primary Response: Initial exposure leads to a slow response.

• Secondary Response: Subsequent exposures trigger a rapid and stronger response.

Branches of Adaptive Immunity

• Cellular Immunity: Mediated by T cells; responsible for destroying infected or abnormal cells.

• Humoral Immunity: Mediated by B cells; responsible for producing antibodies that neutralize pathogens.

• Both branches work together to eliminate antigens and establish immunological memory.

Four Stages of the Adaptive Immune Response

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

2. Lymphocyte Activation: Recognition of antigen activates specific lymphocytes.

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 future responses.

Lymphocytes

T Cells and B Cells

• T cells: Mature in the thymus; include CD4+ helper T cells and CD8+ cytotoxic T cells.

• B cells: Mature in the bone marrow; differentiate into plasma cells that secrete antibodies.

Antigens and Immunogenicity

Key Concepts

• 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 and then lipids.

• Epitope: The specific region of an antigen recognized by lymphocyte receptors.

• Hapten: A small molecule that is not immunogenic by itself but can elicit a response when attached to a carrier protein.

Receptors and Clonal Expansion

Lymphocyte Receptors

• T Cell Receptor (TCR): Recognizes antigen only when presented with Major Histocompatibility Complex (MHC) molecules.

• B Cell Receptor (BCR): Recognizes free (unprocessed) antigen directly.

• Each lymphocyte expresses receptors specific for a single epitope.

Clonal Expansion

• Upon activation, lymphocytes proliferate (clonal expansion) and differentiate into effector cells (combat infection) and memory cells (long-term immunity).

T Cells

Types and Functions

• Helper T cells (CD4+): Coordinate immune responses by activating other immune cells.

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

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

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

• Cytotoxic T cells (CD8+): Destroy infected or abnormal cells by releasing perforins and granzymes.

Major Histocompatibility Complex (MHC)

Classes and Roles

• MHC I: Presents intracellular antigens (e.g., viral proteins) to CD8+ cytotoxic T cells.

• MHC II: Presents extracellular antigens (e.g., bacterial fragments) to CD4+ helper T cells.

Lymphocyte Activation

T Cell Activation

• Requires two signals: (1) Antigen recognition via TCR-MHC interaction, and (2) Co-stimulatory signal from the APC.

B Cell Activation

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

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

Antibodies (Immunoglobulins)

Functions

• Neutralization: Block pathogen binding to host cells.

• Opsonization: Mark pathogens for phagocytosis.

• Agglutination/Precipitation: Clump antigens together for easier removal.

• Complement Activation: Trigger the complement cascade to lyse pathogens.

Structure

• Composed of two heavy chains and two light chains.

• The variable region binds specifically to the antigen's epitope.

Antibody Isotypes

Isotype	Main Function
IgG	Most abundant in serum; provides long-term immunity and memory
IgA	Found in mucosal areas (e.g., gut, respiratory tract); protects mucosal surfaces
IgM	First antibody produced in primary response; effective at agglutination
IgE	Involved in allergic reactions and defense against parasites
IgD	Functions mainly as a B cell receptor

Primary vs. Secondary Immune Response

• Primary Response: Occurs upon first exposure to an antigen; slow, with IgM produced first.

• Secondary Response: Occurs upon re-exposure; rapid and robust, dominated by IgG production.

Self-Tolerance

• Mechanisms that prevent the immune system from attacking the body's own tissues.

• T cells: Tested for self-reactivity in the thymus (negative selection).

• B cells: Tested in the bone marrow; self-reactive cells are eliminated or inactivated.

Types of Immunity

Type	How Immunity is Acquired	Example
Natural Active	Exposure to pathogen through infection	Recovering from chickenpox
Artificial Active	Exposure via vaccination	Receiving the measles vaccine
Natural Passive	Transfer of antibodies from mother to child	Maternal IgG crossing placenta
Artificial Passive	Injection of preformed antibodies (antiserum)	Receiving antivenom after snakebite