Adaptive Immune Response: Humoral and Cell-Mediated Immunity

Adaptive Immune Response

Overview of Humoral vs. Cell-Mediated Immunity

The adaptive immune response is a highly specific defense mechanism that protects the body against pathogens. It is divided into two main branches: humoral immunity and cell-mediated immunity. Each branch involves distinct cell types, mechanisms, and outcomes.

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

• Cell-Mediated Immunity: Mediated by T lymphocytes; effective against intracellular pathogens, such as viruses and some bacteria.

Feature	Humoral Immunity	Cell-Mediated Immunity
Cell Types Involved	B cells, plasma cells	T cells (Helper, Cytotoxic)
Effector Molecules	Antibodies	Cytokines, cytotoxic molecules
Target	Extracellular pathogens	Intracellular pathogens
Effectiveness	Neutralization, opsonization, complement activation	Direct killing of infected cells, activation of macrophages

Types of Acquired Immunity

Acquired immunity can be classified based on how it is obtained:

• Naturally Acquired Immunity:

  • Active: Exposure to pathogens leads to immune response (e.g., infection).

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

• Artificially Acquired Immunity:

  • Active: Vaccination stimulates immune response.

  • Passive: Injection of antibodies (e.g., antiserum).

Antigens (Ag)

Antigens are substances that provoke an immune response. They are typically macromolecules such as proteins, polysaccharides, or lipids found on pathogens.

• Definition: Any substance that can be recognized by the immune system and elicit an immune response.

• Examples: Bacterial toxins, viral proteins, pollen.

• Characteristics: Usually large, complex molecules; contain specific regions called epitopes or antigenic determinants.

• Haptens: Small molecules that are not immunogenic by themselves but can become antigenic when attached to a carrier protein.

• Autoimmune Diseases: Occur when the immune system mistakenly targets self-antigens.

Antibody (Ab) Structure and Function

Antibodies are Y-shaped proteins produced by plasma cells that specifically bind to antigens.

• Structure: Composed of four polypeptide chains (two heavy, two light) linked by disulfide bonds.

• Regions:

  • Variable Region: Forms the antigen-binding site; specificity for antigen.

  • Constant Region: Determines antibody class and function.

  • Fc Region: Made up of constant domains of heavy chains; mediates effector functions (e.g., binding to Fc receptors on cells).

• Antigen Binding Site: Formed by variable regions of both heavy and light chains; located at the tips of the Y-shaped molecule.

• Classes of Antibodies:

  • IgG: Most abundant; crosses placenta; opsonization.

  • IgM: First produced; pentamer; high valency.

  • IgA: Found in mucosal areas; dimer.

  • IgE: Allergic responses; binds to mast cells.

  • IgD: B cell receptor; function not fully understood.

• Valency: Number of antigen-binding sites (e.g., IgG has 2, IgM has 10).

• Results of Ag-Ab Interactions: Neutralization, agglutination, opsonization, complement activation.

Humoral Immunity

Humoral immunity involves B lymphocytes and the production of antibodies to combat pathogens.

• B Lymphocytes:

  • Have surface receptors (BCR) after maturation; proteinaceous in nature.

  • Mature in bone marrow.

• Role in Humoral Response:

  • Recognize antigen via BCR (clonal selection).

  • Proliferate (clonal expansion).

  • Differentiation into plasma cells (produce antibodies) and memory cells (long-lived).

  • Clonal Deletion: Removal of self-reactive B cells; prevents autoimmunity.

• Primary vs. Secondary Humoral Responses:

  • Primary: First exposure; IgM produced; slower response.

  • Secondary: Subsequent exposure; IgG produced; faster, stronger response due to memory cells.

  • Booster Immunization: Enhances memory response.

• T-Dependent vs. T-Independent Responses:

  • T-Dependent: Requires T helper cells; usually protein antigens.

  • T-Independent: Does not require T cells; usually polysaccharide antigens.

• Steps in Humoral Response:

  • T-Independent: B cell binds antigen, proliferates, differentiates.

  • T-Dependent: B cell presents antigen to T helper cell, receives signals, proliferates, differentiates.

Cell-Mediated Immunity

Cell-mediated immunity involves T lymphocytes and is essential for defense against intracellular pathogens.

• Cytokines: Signaling proteins that regulate immune responses; produced by T cells, macrophages, etc.

• T Lymphocytes:

  • Mature in thymus.

  • Have T cell receptors (TCR); recognize antigens only as Ag-MHC complexes.

  • Compare to B cells: T cells do not produce antibodies; B cells do.

• Types of T Cells:

  • Helper T Cells (TH): CD4+; recognize Ag-MHC II on APCs; secrete cytokines to activate other immune cells.

  • Cytotoxic T Cells (TC or CTL): CD8+; recognize Ag-MHC I on target cells; kill infected cells by releasing perforin and granzymes.

• Antigen Recognition: T cells recognize antigens only when presented by MHC molecules on cell surfaces.

• MHC (Major Histocompatibility Complex):

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

  • Class II MHC: Found on professional APCs; presents to CD4+ T cells.

• APC (Antigen Presenting Cells): Cells that process and present antigens with MHC II; include dendritic cells, macrophages, B cells.

Natural Killer (NK) Cells

NK cells are lymphocytes that provide innate immunity by killing infected or abnormal cells.

• Do not have antigen-specific surface receptors.

• Secrete cytotoxic molecules (e.g., perforin, granzymes).

Interrelationship of Humoral and Cell-Mediated Immunity

T helper cells play a central role in linking humoral and cell-mediated immunity. They help B cells respond to T-dependent antigens by providing necessary signals for activation and differentiation.

• T helper cells secrete cytokines that stimulate B cell proliferation and antibody production.

• T-dependent antigens require both B cell and T cell cooperation for effective immune response.

Example:

Vaccination with a protein antigen (e.g., tetanus toxoid) induces a T-dependent humoral response, resulting in the production of high-affinity IgG antibodies and memory cells.