Adaptive Immunity: Humoral and Cellular Responses part 2 (part 1)

Adaptive Immunity

Overview of Adaptive Defenses

The adaptive immune system provides specific defense mechanisms against pathogens through two main branches: humoral immunity (mediated by B cells and antibodies) and cellular immunity (mediated by T cells). These responses are highly specific and involve memory, allowing for a more rapid and effective response upon subsequent exposures to the same antigen.

Humoral Immunity

Clonal Selection of B Cells

Humoral immunity is initiated when B lymphocytes encounter and bind to a specific antigen. This triggers their activation, proliferation, and differentiation into plasma cells and memory B cells.

• Antigen: Any substance that can elicit an immune response by binding to a specific receptor on a B lymphocyte.

• Clonal Selection: Activated B cells proliferate to form a clone of cells, each producing the same antibody specific to the encountered antigen.

• Plasma Cells: Effector B cells that secrete large quantities of antibodies.

• Memory B Cells: Long-lived cells primed to respond rapidly to future exposures of the same antigen.

Primary Response: Occurs upon initial exposure to an antigen, leading to the production of plasma cells and memory B cells. Secondary Response: Upon subsequent exposure, memory B cells rapidly differentiate into plasma cells, resulting in a faster and stronger antibody response.

Antibodies (Immunoglobulins)

Antibodies, also known as immunoglobulins (Igs), are proteins secreted by plasma cells. They constitute the gamma globulin portion of blood and are capable of binding specifically to antigens detected by B cells.

• Structure: Composed of two heavy chains and two light chains, each with variable and constant regions. The antigen-binding site is formed by the variable regions of both chains.

• Classes: Antibodies are grouped into five major classes: IgM, IgA, IgD, IgG, and IgE.

Key Terms: - Variable region: Part of the antibody that binds to the antigen. - Constant region: Determines the class of the antibody and mediates effector functions. - Disulfide bonds: Stabilize the antibody structure.

Antibody Targets and Functions

Antibodies do not directly destroy antigens; instead, they inactivate and tag them for elimination by other immune mechanisms. The formation of antigen-antibody complexes is central to their function.

• Neutralization: Antibodies block specific sites on pathogens, preventing their attachment to host cells.

• Agglutination: Antibodies cause antigens to clump together, facilitating their removal.

• Precipitation: Soluble antigens are cross-linked by antibodies and precipitate out of solution.

• Complement Activation: Antigen-antibody complexes trigger the complement system, leading to cell lysis.

• Enhancement of Phagocytosis: Antibodies mark antigens for destruction by phagocytes.

Active and Passive Humoral Immunity

Humoral immunity can be acquired actively or passively, either naturally or artificially.

Type	Active	Passive
Naturally Acquired	Infection; contact with pathogen	Antibodies passed from mother to fetus via placenta, or to infant in her milk
Artificially Acquired	Vaccine; dead or attenuated pathogens	Injection of exogenous antibodies (gamma globulin)

Cellular Immune Response

Overview of T Cell-Mediated Immunity

Cellular immunity is provided by T lymphocytes, which defend against intracellular antigens. Some T cells directly kill infected cells, while others release chemicals that regulate the immune response.

• Helper T Cells (TH): Activate B cells, other T cells, and macrophages; direct the adaptive immune response.

• Regulatory T Cells: Moderate the immune response to prevent overactivity.

• Cytotoxic T Cells (TC): Destroy cells harboring foreign antigens.

• Memory T Cells: Provide long-term immunity by responding rapidly to previously encountered antigens.

Naive T Cells: T cells that have not yet encountered their specific antigen are termed naive CD4 or CD8 cells. Upon activation, they differentiate into helper, cytotoxic, or regulatory T cells.

MHC Proteins and Antigen Presentation

T cells respond only to processed fragments of antigens displayed on the surfaces of cells, a process known as antigen presentation. This is vital for the activation of naive T cells and the normal functioning of effector T cells.

• Class I MHC Proteins: Displayed by all cells except red blood cells; present endogenous antigens (from within the cell). Crucial for CD8 cell activation.

• Class II MHC Proteins: Displayed by antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells; present exogenous antigens (from outside the cell) that have been engulfed and processed. Recognized by helper T cells.

T Cell Activation, Proliferation, and Differentiation

T cell activation is a two-step process involving antigen binding and co-stimulation, both occurring on the surface of the same antigen-presenting cell (APC). Both steps are required for clonal selection and the development of effector and memory T cells.

• Primary T Cell Response: Peaks within a week of antigen exposure.

• Apoptosis: Activated T cells undergo apoptosis between days 7 and 30 to prevent excessive inflammation and potential tissue damage.

• Memory T Cells: Persist after the initial response and mediate secondary responses upon re-exposure to the antigen.

Benefit of Apoptosis: Prevents hyperplasia and cancer by eliminating excess activated T cells that produce large amounts of inflammatory cytokines.

Key Terms and Concepts

• Antigen: Any substance that can trigger an immune response.

• Antibody: Protein produced by B cells that binds specifically to antigens.

• Plasma Cell: Effector B cell that secretes antibodies.

• Memory Cell: Long-lived cell that responds rapidly to subsequent exposures to the same antigen.

• MHC (Major Histocompatibility Complex): Proteins on cell surfaces that present antigen fragments to T cells.

• APC (Antigen-Presenting Cell): Cell that processes and presents antigens to T cells (e.g., dendritic cells, macrophages, B cells).

Formulas and Equations

• Antigen-Antibody Complex Formation:

$\text{Antigen} + \text{Antibody} \rightarrow \text{Antigen-Antibody Complex}$

• Clonal Expansion:

$\text{Activated Lymphocyte} \xrightarrow{\text{Proliferation}} \text{Clone of Identical Cells}$

Additional info: The notes have been expanded to provide definitions, context, and examples for key terms and processes in adaptive immunity, suitable for college-level Anatomy & Physiology students.