Chapter 12: Adaptive Immunity – Mini-Textbook Study Notes

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Adaptive Immunity

Overview of Adaptive Immunity

The adaptive immune system is a highly specialized defense mechanism that provides specific and long-lasting protection against pathogens. Unlike innate immunity, adaptive responses take longer to develop but are highly specific to particular antigens and generate immunological memory, allowing for rapid and effective responses upon subsequent exposures.

Primary exposure: Response takes several days to a week; symptoms may occur.
Secondary exposure: Response is rapid and often prevents symptoms.
Two main branches: Cellular (T cell–mediated) and Humoral (antibody–mediated) immunity.

General Steps in an Adaptive Immune Response

Four Main Stages

Both cellular and humoral responses follow these stages to eliminate antigens and establish memory:

Antigen Presentation
Lymphocyte Activation
Lymphocyte Proliferation and Differentiation
Antigen Elimination and Memory

Cells of Adaptive Immunity

Lymphocytes: T Cells and B Cells

Lymphocytes are the primary leukocytes of adaptive immunity:

T cells: Produced in bone marrow, mature in thymus; involved in both cellular and humoral immunity.
B cells: Produced and mature in bone marrow; coordinate humoral response by producing antibodies.
Each lymphocyte expresses thousands of identical receptors, each specific for a single epitope.

Antigen Features

Definition and Properties

An antigen is any substance that can trigger an immune response. The immune system's response depends on the antigen's size, molecular complexity, and chemical composition. Most antigens are proteins or polysaccharides from pathogens.

Haptens: Incomplete antigens that require attachment to a larger molecule to elicit a response.
Epitopes: Specific regions of an antigen recognized by B or T cell receptors.

Diagram showing antigens on a pathogen, with multiple epitopes on an antigen

Receptors and Antigen Recognition

TCRs and BCRs

Each T cell receptor (TCR) or B cell receptor (BCR) is specific for a single epitope. The diversity of these receptors is generated by gene shuffling, allowing the immune system to recognize virtually any antigen.

TCR: Recognizes one epitope; involved in cellular immunity.
BCR: Recognizes one epitope; involved in humoral immunity.

Self-Tolerance in Lymphocytes

Screening for Self-Tolerance

To prevent autoimmunity, T and B cells undergo screening to ensure they do not react to self-antigens:

T cells: Screened in the thymus for recognition of self-MHC and non-reactivity to self-antigens.
B cells: Screened in the bone marrow to prevent production of self-reactive antibodies.
Cells failing self-tolerance undergo apoptosis.

Major Histocompatibility Complexes (MHC)

Classes and Functions

MHC molecules are glycoproteins on cell membranes that present antigenic epitopes to T cells. There are two main classes:

MHC I: Present on all nucleated cells (except RBCs); identifies cells as self and presents intracellular antigens to cytotoxic T cells.
MHC II: Present only on antigen-presenting cells (APCs) such as macrophages, B cells, and dendritic cells; presents extracellular antigens to helper T cells.

Diagram comparing MHC I and MHC II on cell membranes

Antigen Location and MHC Presentation

Intracellular antigens: Presented by MHC I (e.g., viral proteins, cancer cell antigens).
Extracellular antigens: Presented by MHC II (e.g., bacterial, fungal, or parasitic antigens).

Cellular Response of Adaptive Immunity

Stages of T Cell Response

Stage 1: Antigen Presentation – Intracellular antigens are presented by MHC I to cytotoxic T cells; extracellular antigens are presented by MHC II to helper T cells.
Stage 2: T Cell Activation – Requires two signals: TCR-MHC interaction and co-stimulatory signals.
Stage 3: Proliferation and Differentiation – Activated T cells proliferate and differentiate into effector and memory cells.
Stage 4: Antigen Elimination – Cytotoxic T cells induce apoptosis in infected or abnormal cells; helper T cells coordinate immune responses via cytokines.

MHC-I Presenting Intracellular Antigens

Intracellular antigens are processed and presented by MHC I to cytotoxic T cells, which then determine if the cell is self or infected.

Diagram of MHC-I presenting intracellular antigens to cytotoxic T cells

MHC-II Presenting Extracellular Antigens

APCs phagocytize extracellular antigens, process them, and present them via MHC II to helper T cells.

Diagram of MHC-II presenting extracellular antigens to helper T cells

Antigen Elimination by T Cells

Cytotoxic T cells release perforins and granzymes to induce apoptosis in target cells. Helper T cells release cytokines to activate other immune cells.

Diagram showing cytotoxic T cell killing a target cell via perforins and granzymes Diagram showing helper T cell releasing cytokines to coordinate immune response

Humoral Response of Adaptive Immunity

Stages of B Cell Response

Stage 1: Antigen Presentation – B cells can be activated by T-dependent or T-independent antigens.
Stage 2: B Cell Activation – T-dependent activation requires help from T helper cells; T-independent activation occurs with repetitive antigens.
Stage 3: Proliferation and Differentiation – Activated B cells proliferate and differentiate into plasma cells (antibody producers) and memory B cells.
Stage 4: Antigen Elimination – Antibodies neutralize, opsonize, and agglutinate antigens, and activate complement.

T-Dependent B Cell Activation

Most antigens require T helper cell assistance for full B cell activation. The process involves antigen binding, processing, MHC II presentation, and cytokine signaling from T helper cells.

Diagram of T-dependent B cell activation by T helper cell

B Cell Proliferation and Differentiation

Activated B cells proliferate and differentiate into plasma cells (which secrete antibodies) and memory B cells (which provide long-term immunity).

Diagram of B cell proliferation and differentiation into plasma and memory cells

Antibody Structure and Isotypes

Antibody Structure

Antibodies (immunoglobulins) are Y-shaped molecules composed of two heavy and two light chains, each with variable and constant regions. The variable region forms the antigen-binding site, while the constant region determines the antibody class.

Antibody Isotypes

There are five main antibody isotypes, each with distinct functions and locations:

Isotype	Structure	Main Features
IgG	Monomer	Most abundant in serum; crosses placenta; produced second in primary infection
IgA	Dimer/Monomer	Most abundant in mucosal secretions; protects mucosal surfaces
IgM	Pentamer/Monomer	First produced in primary infection; largest; effective in blood
IgD	Monomer	B cell receptor; regulatory role
IgE	Monomer	Lowest in serum; involved in allergic reactions and defense against parasites

Diagram of antibody isotypes: IgG, IgA, IgM, IgD, IgE

Immunological Memory

Primary vs. Secondary Response

Primary exposure to an antigen generates IgM first, then IgG. Secondary exposure triggers a rapid, high-affinity IgG response due to memory B and T cells, providing long-term protection.

Acquisition of Humoral Immunity

Types of Adaptive Immunity

Naturally acquired active immunity: Infection triggers immune response; long-term protection.
Naturally acquired passive immunity: Antibodies received (e.g., maternal); temporary protection.
Artificially acquired active immunity: Vaccination; long-term protection.
Artificially acquired passive immunity: Antibody administration (antiserum); temporary protection.

Additional info: The adaptive immune system's ability to generate a vast diversity of antigen receptors is due to somatic recombination and hypermutation processes in lymphocytes. Memory cells are the basis for the effectiveness of vaccines and long-term immunity.