Adaptive Immunity: Specific Defenses of the Host – Study Notes

Adaptive Immunity: Specific Defenses of the Host

Overview of Adaptive Immunity

Adaptive immunity is a highly specific defense mechanism that targets particular pathogens. It is acquired through exposure to antigens via infection or vaccination. The immune system's response to a pathogen is classified as either a primary response (first exposure) or a secondary response (subsequent exposures), with the latter being faster and more effective due to immunological memory.

• Primary response: Initial encounter with an antigen.

• Secondary response: Enhanced response upon re-exposure, mediated by memory cells.

Dual Nature of the Adaptive Immune System

The adaptive immune system consists of two main branches: humoral immunity and cellular immunity. Each branch targets different types of pathogens and utilizes distinct mechanisms.

• Humoral immunity: Mediated by B cells and antibodies; combats extracellular pathogens such as bacteria and toxins.

• Cellular immunity: Mediated by T cells; targets intracellular pathogens like viruses, some fungi, and parasites.

Humoral Immunity

B cells are lymphocytes that originate and mature in the red bone marrow. They recognize antigens and produce antibodies, which are proteins that specifically bind to antigens and neutralize them.

• B cells: Recognize antigens and differentiate into plasma cells (produce antibodies) and memory cells.

• Antibodies: Bind to antigens and facilitate their removal from the body.

Cellular Immunity

T cells mature in the thymus and are responsible for recognizing antigenic peptides presented by phagocytic cells. T cell receptors (TCRs) interact with antigens, leading to the secretion of cytokines.

• T helper cells (TH): Express CD4+; assist other immune cells via cytokine signaling.

• T cytotoxic cells (TC): Express CD8+; directly kill infected cells.

• Natural killer (NK) cells: Destroy cells without prior activation.

Cytokines: Chemical Messengers of Immune Cells

Cytokines are signaling molecules produced by immune cells in response to stimuli. They regulate immune responses and mediate communication between cells.

• Interleukins: Communication between leukocytes.

• Chemokines: Induce migration of leukocytes.

• Interferons (IFNs): Inhibit viral replication.

• Tumor necrosis factor (TNF): Mediates inflammation.

• Hematopoietic cytokines: Regulate blood cell formation.

Antigens and Antibodies

Antigens, Epitopes, and Haptens

Antigens are substances that provoke an immune response, typically components of pathogens. Antibodies interact with specific regions on antigens called epitopes. Haptens are small molecules that become antigenic only when attached to larger carrier molecules.

• Antigen: Any substance that induces antibody production.

• Epitope: Specific region of an antigen recognized by antibodies.

• Hapten: Small molecule that is antigenic only when attached to a carrier.

Antibody Structure and Function

Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins composed of two identical light chains and two identical heavy chains. The variable regions at the tips of the Y bind to epitopes, while the constant (Fc) region determines the antibody class.

• Valence: Number of antigen-binding sites (bivalent antibodies have two).

• Classes of antibodies: IgG, IgM, IgA, IgD, IgE.

Functions of Antibodies

Antibodies protect the host by binding to antigens and facilitating their destruction through various mechanisms:

• Agglutination: Clumping of pathogens.

• Opsonization: Coating antigens to enhance phagocytosis.

• Neutralization: Blocking pathogen attachment to host cells.

• Activation of complement: Triggering cell lysis and inflammation.

• Antibody-dependent cell-mediated cytotoxicity: Destruction by immune cells.

Immunoglobulin Classes

Immunoglobulins are classified based on their structure and function. Each class has distinct roles in the immune response.

Class	Valence	Main Function
IgG	2	Most abundant; crosses placenta; protects against bacteria and viruses
IgM	10	First antibody produced; effective in agglutination
IgA	4	Found in mucosal areas; protects body surfaces
IgD	2	Functions mainly as a B cell receptor
IgE	2	Involved in allergic responses and defense against parasites

Humoral Immunity Response Process

Clonal Selection and Differentiation

B cells undergo clonal selection upon encountering an antigen. The process involves antigen presentation via MHC class II molecules, activation by T helper cells, and differentiation into plasma cells and memory cells.

• Plasma cells: Produce antibodies.

• Memory cells: Provide long-term immunity.

• Clonal deletion: Removes self-reactive B cells.

Major Histocompatibility Complex (MHC)

MHC molecules are essential for antigen presentation and self-recognition. Class I MHC is present on all nucleated cells, while class II MHC is found on antigen-presenting cells.

• MHC I: Presents endogenous antigens; recognized by CD8+ T cells.

• MHC II: Presents exogenous antigens; recognized by CD4+ T cells.

Cellular Immunity Response Process

Antigen-Presenting Cells (APCs)

APCs such as dendritic cells and macrophages process and present antigens to T cells, initiating cellular immune responses.

• Dendritic cells: Engulf and degrade microbes; present antigens to T cells.

• Macrophages: Activated by cytokines; migrate to lymph tissue to present antigens.

Classes of T Cells

T cells are classified based on their surface markers and functions:

• CD4+ T helper cells (TH): Assist B cells and other immune cells.

• CD8+ cytotoxic T lymphocytes (CTL): Kill infected cells.

• T regulatory cells (Treg): Suppress immune responses against self.

T Helper Cell Subtypes

T helper cells differentiate into subtypes based on cytokine production:

• TH1: Activate macrophages and CD8+ T cells; promote phagocytosis.

• TH2: Activate B cells to produce IgE; important in allergic responses.

• TH17: Produce IL-17; recruit neutrophils for protection against extracellular pathogens.

T Cytotoxic Cells (CD8+ T Cells) and Apoptosis

CTLs recognize and kill infected or altered self-cells by inducing apoptosis, a programmed cell death process that prevents the spread of infection.

• Apoptosis: Cell death characterized by DNA fragmentation and membrane blebbing.

Natural Killer (NK) Cells

NK cells are granular leukocytes that destroy cells lacking MHC class I self-antigens. They kill virus-infected and tumor cells, often without prior antigen stimulation.

• Function: Form pores in target cells, leading to lysis or apoptosis.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

ADCC is a mechanism by which immune cells destroy large pathogens, such as protozoans and helminths, that are too large to be phagocytized. Antibodies coat the target cell, and immune cells attach to the Fc region, releasing chemicals to lyse the cell.

Immunological Memory

Primary vs. Secondary Immune Response

Immunological memory enables the immune system to respond more rapidly and effectively upon subsequent exposures to the same antigen. The antibody titer reflects the intensity of the humoral response.

• Primary response: IgM produced first, followed by IgG.

• Secondary response: More rapid, greater magnitude, dominated by IgG.

Types of Adaptive Immunity

Four Types of Adaptive Immunity

Adaptive immunity can be acquired naturally or artificially, and can be active or passive:

• Naturally acquired active immunity: Resulting from infection.

• Naturally acquired passive immunity: Transfer of antibodies from mother to child.

• Artificially acquired active immunity: Vaccination.

• Artificially acquired passive immunity: Injection of antibodies.