The Adaptive Immune Response: Humoral and Cell-Mediated Immunity

The Adaptive Immune Response

Overview of Adaptive Immunity

The adaptive immune response is a specific defense mechanism that develops after exposure to antigens. It involves two main branches: humoral immunity and cell-mediated immunity, each with distinct roles, cell types, and mechanisms.

• Humoral Immunity: Mediated by B lymphocytes and antibodies; primarily defends against extracellular pathogens.

• Cell-Mediated Immunity: Mediated by T lymphocytes; primarily defends against intracellular pathogens and abnormal cells.

Humoral vs. Cell-Mediated Immunity

Comparison of Humoral and Cell-Mediated Immunity

• Humoral Immunity:

  • Cell Types: B lymphocytes (B cells)

  • Effector Molecules: Antibodies (immunoglobulins)

  • Targets: Extracellular pathogens (bacteria, viruses in body fluids)

  • Mechanism: Antibodies bind to antigens, neutralizing or marking them for destruction.

• Cell-Mediated Immunity:

  • Cell Types: T lymphocytes (Helper T cells, Cytotoxic T cells)

  • Effector Molecules: Cytokines, cytotoxic granules

  • Targets: Intracellular pathogens (viruses, some bacteria), cancer cells, transplanted tissues

  • Mechanism: Direct killing of infected cells or activation of other immune cells.

Types of Acquired Immunity

Classification and Examples

• Naturally Acquired Immunity:

  • Active: Immunity developed after exposure to antigens (e.g., recovery from infection)

  • Passive: Transfer of antibodies from mother to child (e.g., via placenta or breast milk)

• Artificially Acquired Immunity:

  • Active: Immunity developed after vaccination (e.g., immunization with attenuated pathogens)

  • Passive: Injection of preformed antibodies (e.g., antitoxins, immune globulin therapy)

Antigens and Related Concepts

Antigens (Ag)

• Definition: Substances that provoke an immune response by stimulating the production of antibodies or sensitized T cells.

• Examples: Proteins, polysaccharides, lipids, nucleic acids (most potent antigens are proteins).

• Characteristics: Usually large, complex macromolecules; possess specific regions called epitopes.

• Role in Immune Response: Recognized as foreign; trigger antibody or T cell responses.

• Autoimmune Diseases: Occur when the immune system reacts against self-antigens.

Haptens

• Definition: Small molecules that are not immunogenic by themselves but can elicit an immune response when attached to a larger carrier molecule.

• Examples: Penicillin (can cause allergic reactions when bound to proteins).

Antigenic Determinants (Epitopes)

• Definition: Specific regions on an antigen recognized by antibodies or T cell receptors.

Antibodies (Immunoglobulins, Ab)

Structure and Function

• Definition: Y-shaped proteins produced by B cells that specifically bind to antigens.

• Structure:

  • Composed of four polypeptide chains: two identical heavy chains and two identical light chains, linked by disulfide bonds.

  • Each chain has a variable region (binds antigen) and a constant region (determines class and function).

  • Fc Region: Formed by the constant regions of the heavy chains; mediates interactions with immune cells and complement.

  • Antigen Binding Site: Formed by the variable regions of both heavy and light chains at the tips of the "Y"; each antibody has two identical binding sites.

Classes of Antibodies

Class	Main Function	Valency
IgG	Main antibody in serum; crosses placenta; opsonization, neutralization	2
IgM	First antibody produced; effective in agglutination and complement activation	10 (pentamer)
IgA	Secretory antibody; found in mucosal areas, saliva, tears, breast milk	4 (dimer)
IgD	Functions mainly as B cell receptor	2
IgE	Involved in allergic reactions and defense against parasites	2

Results of Antigen-Antibody Interactions

• Neutralization of toxins and pathogens

• Agglutination and precipitation of antigens

• Opsonization (enhanced phagocytosis)

• Activation of complement system

• Antibody-dependent cell-mediated cytotoxicity (ADCC)

Humoral Immunity

B Lymphocytes (B Cells)

• Characteristics: Mature in bone marrow; express B cell receptors (BCRs) on their surface, which are membrane-bound antibodies.

• Role in Humoral Response:

  • Recognize specific antigens via BCR (clonal selection)

  • Proliferate (clonal expansion) and differentiate into plasma cells (antibody-secreting) and memory B cells (long-lived, rapid response upon re-exposure)

  • Clonal Deletion: Removal of self-reactive B cells during development to prevent autoimmunity

Primary vs. Secondary Humoral Responses

• Primary Response: First exposure to antigen; slower, mainly IgM produced initially, followed by IgG.

• Secondary Response: Subsequent exposures; faster and stronger, mainly IgG due to memory B cells.

• Booster Immunization: Increases memory cell pool and antibody levels.

T-Dependent vs. T-Independent Humoral Responses

• T-Dependent Antigens: Require help from T helper cells; usually proteins; result in strong, long-lasting immunity and memory cell formation.

• T-Independent Antigens: Can activate B cells without T cell help; usually polysaccharides; weaker response, little to no memory.

Steps in Humoral Response

1. B cell binds antigen via BCR.

2. For T-dependent antigens: B cell presents antigen on MHC II to T helper cell; T helper cell provides cytokine signals.

3. B cell proliferates and differentiates into plasma and memory cells.

4. For T-independent antigens: B cell is directly activated and differentiates, but response is weaker.

Cell-Mediated Immunity

Cytokines

• Definition: Small proteins released by cells that regulate the intensity and duration of immune responses.

• Roles: Mediate communication between immune cells, stimulate cell proliferation, differentiation, and activation.

T Lymphocytes (T Cells)

• Characteristics: Mature in thymus; express T cell receptors (TCRs) on their surface; recognize antigen only when presented with MHC molecules.

• Comparison with B Cells:

  • B cells recognize free antigens; T cells recognize processed antigens presented by MHC.

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

  • B cells produce antibodies; T cells mediate cellular responses.

Types of T Cells and Their Functions

Type	Surface Marker	Main Function
Helper T cells (TH)	CD4	Activate B cells, other T cells, and macrophages via cytokine secretion
Cytotoxic T cells (TC or CTL)	CD8	Directly kill infected or abnormal cells

Antigen Recognition by T Cells

• T cells recognize antigens only as peptide fragments bound to MHC molecules on cell surfaces.

• TCR and CD4/CD8 coreceptors are required for recognition.

Major Histocompatibility Complex (MHC)

• Definition: Group of genes encoding cell surface proteins essential for antigen presentation.

• Classes:

  • MHC I: Found on all nucleated cells; present endogenous antigens to CD8+ T cells.

  • MHC II: Found on antigen-presenting cells (APCs); present exogenous antigens to CD4+ T cells.

Antigen-Presenting Cells (APCs)

• Role: Process and present antigens with MHC II to helper T cells.

• Examples: Dendritic cells, macrophages, B cells.

CD4+ and CD8+ T Cells

• CD4+ T Cells (Helper T Cells): Recognize antigen-MHC II complexes on APCs; differentiate into effector and memory cells; secrete cytokines to activate other immune cells.

• CD8+ T Cells (Cytotoxic T Cells): Recognize antigen-MHC I complexes on target cells; differentiate into cytotoxic T lymphocytes (CTLs) and memory cells; secrete perforin and granzymes to induce apoptosis in target cells (e.g., virus-infected or cancer cells).

Natural Killer (NK) Cells

• Definition: Lymphocytes that can kill virus-infected and tumor cells without antigen-specific receptors.

• Surface Receptors: Lack antigen-specific receptors; recognize abnormal cells via other mechanisms.

• Secretions: Release perforin and granzymes to induce apoptosis in target cells.

Interrelationship of Humoral and Cell-Mediated Immunity

• T helper cells (CD4+) play a central role in both branches by providing cytokine help to B cells (for antibody production) and activating cytotoxic T cells and macrophages.

• T-dependent antigens require T helper cell involvement for optimal antibody responses and memory formation.

Example:

Vaccination with an inactivated virus stimulates both humoral (antibody production) and cell-mediated (cytotoxic T cell) responses, providing comprehensive immunity.

Additional info: The above notes expand on the study guide outline by providing definitions, mechanisms, and examples for each concept, ensuring a self-contained and exam-ready summary.