lec 26:Adaptive Immunity: Mechanisms, Components, and Clinical Relevance

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Adaptive Immunity: Overview

Definition and Core Functions

Adaptive immunity is a highly specific, acquired defense system in vertebrates that targets particular pathogens (antigens), eliminates them, and retains memory for future protection. Unlike innate immunity, adaptive immunity utilizes B cells and T cells to generate a tailored response, providing long-lasting and highly specific defense.

Antibody production: B cells generate antibodies to neutralize pathogens (humoral immunity).
Cell-mediated killing: T cells destroy infected cells and coordinate immune responses.
Immunological memory: Faster, stronger response upon re-exposure to the same pathogen.

Key Components and Mechanisms

Lymphocyte Development and Differentiation

Lymphocytes, the core cells of adaptive immunity, originate from stem cells in the bone marrow. They differentiate into B cells (mature in bone marrow) and T cells (mature in thymus), each with specialized roles in immune defense.

Hematopoietic stem cell differentiation into myeloid and lymphoid lineages

B Cells and Humoral Immunity

B cells mature in the bone marrow and are responsible for humoral immunity. They produce antibodies that directly neutralize antigens in body fluids.

Bone marrow anatomy and blood cell development

T Cells and Cell-Mediated Immunity

T cells mature in the thymus and are essential for cell-mediated immunity. They kill infected cells and coordinate the overall immune response.

Anatomy of the thymus gland

Antigen Recognition and Immune Memory

Adaptive immunity relies on the ability to distinguish self from non-self, targeting specific antigens on pathogens. Upon infection, some T and B cells become memory cells, providing long-term protection against future infections by the same pathogen.

Main Features of Adaptive Immunity

Specificity, Versatility, Memory, and Tolerance

Specificity: Antibodies and receptors are uniquely designed to bind a specific antigen, similar to a lock and key.
Versatility: The system can recognize and respond to millions of different antigens.
Memory: Previous infections are remembered, enabling rapid and robust responses upon re-exposure.
Tolerance: The immune system differentiates self from non-self, preventing attacks on the body's own tissues.

Types of adaptive immunity: humoral and cell-mediated Immune tolerance and autoimmunity

The Adaptive Immune Response Process

Stages of the Adaptive Response

Antigen Detection: Dendritic cells capture antigens and present them to T cells in lymph nodes.
Activation: T cells are activated and subsequently activate B cells.
Expansion and Differentiation: B cells proliferate and differentiate into plasma cells that secrete antibodies.
Targeted Destruction: Antibodies and T cells eliminate the pathogen.
Memory Formation: Long-lived memory B and T cells remain for future protection.

Dendritic cell presenting antigen to T cells T cell activation and B cell activation B cell differentiation into plasma cells Effector and memory T cell formation Memory cell persistence

Antibody Structure and Synthesis

Antibody Structure

Antibodies (immunoglobulins) are Y-shaped proteins produced by B cells. Each antibody consists of four polypeptide chains (two heavy and two light) held together by disulfide bonds. The variable region at the tips of the Y forms the antigen-binding site, while the constant region determines the antibody's class and function.

Antibody structure diagram Antibody chains and disulfide bonds Heavy and light chains of antibody Variable region and antigen binding site Constant region and Fc fragment

Synthesis and Secretion

Activation: Naïve B cells encounter their specific antigen and receive help from Helper T cells.
Differentiation: Activated B cells proliferate and mature into plasma cells, which are specialized for antibody production.
Production: Plasma cells can secrete thousands of antibody molecules per second.
Class Switching: B cells initially produce IgM, but can switch to other classes (IgG, IgA, etc.) depending on the immune context.

B cell activation and antibody production B cell activation and differentiation Plasma cell antibody secretion Antibody class switching and isotypes

Classes of Antibodies (Immunoglobulins)

Overview of Antibody Isotypes

Class	Main Function	Structure
IgG	Opsonization, neutralization, secondary response	Monomer
IgM	Agglutination, complement activation, primary response	Pentamer
IgA	Mucosal defense, found in secretions	Dimer (secretory form)
IgE	Allergic responses, binds mast cells/basophils	Monomer
IgD	B cell receptor, rarely secreted	Monomer

IgG antibody structure and function IgM pentamer structure IgA dimer structure IgE antibody and allergic response IgD antibody on B cell surface

Primary vs. Secondary Immune Response

Antibody Kinetics During Infection

Upon first exposure to a pathogen, the adaptive immune system produces a primary response characterized by a slow rise in IgM, followed by IgG. Upon subsequent exposures, the secondary response is faster and more robust, dominated by high-affinity IgG antibodies.

Primary and secondary immune response graph Antibody kinetics: IgM and IgG over time

Coordination Between Humoral and Cellular Immunity

Role of Helper T Cells (CD4+)

Helper T cells act as coordinators, linking the cellular and humoral branches of adaptive immunity. They recognize antigens presented by antigen-presenting cells (APCs) and provide essential signals (cytokines) to activate B cells and promote antibody production, class switching, and memory formation.

Helper T cell linking humoral and cellular immunity T cell and B cell interaction via MHC and cytokines

Cytotoxic T Cells (CD8+)

Cytotoxic T cells are responsible for killing infected or abnormal cells by inducing apoptosis. They recognize antigens presented on MHC Class I molecules and are crucial for controlling intracellular infections, such as viruses.

Cytotoxic T cell killing infected cell

Major Histocompatibility Complex (MHC)

Structure and Function

MHC molecules (called HLA in humans) are cell surface glycoproteins that present peptide fragments to T cells, enabling the immune system to distinguish self from non-self. There are two main classes:

MHC Class I: Present on all nucleated cells; present endogenous antigens to CD8+ cytotoxic T cells.
MHC Class II: Present on antigen-presenting cells (APCs); present exogenous antigens to CD4+ helper T cells.

MHC Class I vs MHC Class II structure

Key Concepts in MHC Biology

MHC Restriction: T cells recognize antigens only when presented by self-MHC molecules.
Extreme Polymorphism: High genetic diversity ensures population-level protection against diverse pathogens.
Codominant Expression: Both maternal and paternal MHC alleles are expressed, increasing antigen presentation diversity.
Cross-Presentation: Dendritic cells can present exogenous antigens on MHC Class I to activate cytotoxic T cells.
Clinical Relevance: MHC molecules are central to organ transplantation compatibility and are linked to autoimmune diseases.

Summary Table: Adaptive Immunity Key Points

Component	Origin/Maturation	Main Function
B cell	Bone marrow	Antibody production (humoral immunity)
T cell	Thymus	Cell-mediated immunity, coordination
Antibody	B cell/plasma cell	Neutralization, opsonization, complement activation
MHC I	All nucleated cells	Present to CD8+ T cells
MHC II	APCs	Present to CD4+ T cells

Additional info: This guide covers the essential mechanisms, cell types, and clinical relevance of adaptive immunity, suitable for exam preparation in a college-level microbiology course.