Chapter 17: Adaptive Immunity – Specific Defenses of the Host

Specific Defenses of the Host: The Immune Response

Introduction

The adaptive immune system provides specific defense mechanisms against pathogens, distinguishing self from non-self and generating immunological memory. This chapter focuses on the components, mechanisms, and interactions of humoral and cellular immunity.

The Adaptive Immune System

Overview

• Adaptive (Specific) Immunity is characterized by specificity and memory, targeting particular pathogens and retaining information for future responses.

• It is divided into humoral immunity (mediated by antibodies produced by B cells) and cellular immunity (mediated by T cells).

Types of Acquired Immunity

Acquired Immunity	Description
Naturally Acquired Active Immunity	Immunity developed after exposure to antigens in daily life (e.g., infection).
Artificially Acquired Active Immunity	Immunity developed after exposure to antigens via vaccination.
Naturally Acquired Passive Immunity	Immunity acquired by transfer of antibodies from mother to child (e.g., via placenta or breast milk).
Artificially Acquired Passive Immunity	Immunity acquired by injection of antibodies (e.g., antiserum).

Duality of the Adaptive Immune System

Humoral (Antibody-Mediated) Immunity

• B lymphocytes defend against extracellular antigens such as bacteria and toxins.

• Antibodies circulate in body fluids and bind to antigens, neutralizing or marking them for destruction.

Cell-Mediated Immunity

• T lymphocytes defend against intracellular antigens, including viruses, fungi, parasites, and some bacteria.

• T cells also play a role in tumor surveillance and transplant rejection.

Antigens and Antibodies

Antigens (Immunogens)

• Antigen: Any substance that can induce an immune response, typically a component of a microbe.

• Epitope (Antigenic Determinant): The specific region of an antigen recognized by antibodies. Each antigen may have multiple epitopes.

• Hapten: A small molecule that is not immunogenic by itself but can elicit an immune response when attached to a carrier protein.

Antibody (Immunoglobulin) Structure

• Antibodies are Y-shaped molecules composed of two heavy chains and two light chains, linked by disulfide bonds.

• Each antibody has a variable region (antigen-binding site) and a constant region (Fc region).

Immunoglobulin Classes

Type	Structure	Not found after birth	Location	Functions
IgG	Monomer	No	Blood, lymph, intestine	Main antibody in secondary response; crosses placenta; opsonization; neutralization
IgM	Pentamer	No	Blood, lymph, B cell surface	Main antibody in primary response; agglutination; complement activation
IgA	Dimer	No	Secretions (saliva, tears, mucus, breast milk)	Mucosal protection
IgD	Monomer	No	B cell surface, blood, lymph	Initiates B cell response
IgE	Monomer	No	Bound to mast cells, basophils	Allergic reactions; defense against parasites

Components of the Lymphatic System

• Primary lymphoid tissues: Bone marrow and thymus (sites of lymphocyte development).

• Secondary lymphoid tissues: Spleen, lymph nodes, and lymphoid tissues (sites of immune response initiation).

Differentiation of B and T Cells

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

• Both originate from stem cells in bone marrow.

B Cells and Humoral Immunity

Clonal Selection and Differentiation

• B cells are activated when their immunoglobulin receptors bind to specific antigens.

• Activated B cells proliferate and differentiate into plasma cells (antibody-producing) and memory B cells.

• Clonal selection ensures specificity and memory in the humoral response.

Antigen-Antibody Binding

• Binding of antibody to antigen forms an antigen-antibody complex, leading to pathogen neutralization, opsonization, agglutination, and complement activation.

Immunological Memory

• Primary response: Slow, occurs after first exposure to antigen; mainly IgM produced.

• Secondary response: Rapid and robust upon subsequent exposures; mainly IgG produced.

Monoclonal Antibodies

• Monoclonal antibodies are produced by hybridoma cells (fusion of B cells and myeloma cells) and are specific to a single epitope.

• Applications include diagnostics, therapeutics, and research.

T Cells and Cell-Mediated (Cellular) Immunity

Chemical Messengers: Cytokines

Cytokine	Representative Activity
IL-4	Involved in differentiation of CD4 T helper cells
IL-2	Activates macrophages; increases cytotoxicity
Gamma-IFN	Activates macrophages; increases antigen presentation
TNF	Inflammatory reactions; autoimmune responses

Cellular Components

• T cells respond to intracellular antigens, including viruses and some bacteria.

• Helper T cells (CD4+) activate other immune cells; cytotoxic T cells (CD8+) destroy infected cells.

• T cells recognize antigens presented by antigen-presenting cells (APCs) in association with MHC molecules.

Classes of T Cells and Their Functions

Type	Function
Helper T cell (Th)	Activate B cells, cytotoxic T cells, and macrophages
Cytotoxic T lymphocyte (CTL)	Destroy virus-infected and tumor cells
Regulatory (Suppressor) T cell (Treg)	Suppress immune response to prevent autoimmunity

Nonspecific Cellular Components

• Activated macrophages: Phagocytose pathogens and present antigens to T cells.

• Natural killer (NK) cells: Destroy virus-infected and tumor cells without prior sensitization.

• Antibody-dependent cell-mediated cytotoxicity (ADCC): Involves killing of antibody-coated target cells by immune effector cells.

Interrelationship of Cellular and Humoral Immunity

• Some antigens (T-independent) can activate B cells directly; others (T-dependent) require T cell help.

• Humoral and cellular immunity function interdependently to protect against diverse pathogens.

Key Terms and Concepts

• Antigen: Substance that induces an immune response.

• Epitope: Specific region of antigen recognized by antibody.

• Immunoglobulin: Antibody molecule.

• Clonal selection: Process by which lymphocytes proliferate in response to antigen.

• Memory cell: Long-lived lymphocyte that responds rapidly upon re-exposure to antigen.

• Cytokine: Protein messenger that regulates immune responses.

• MHC (Major Histocompatibility Complex): Molecules that present antigens to T cells.

Important Equations and Diagrams

• Antigen-Antibody Binding:

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

• Immunological Memory Graph:

$\text{Primary Response: Slow, mainly IgM} \qquad \text{Secondary Response: Fast, mainly IgG}$

Summary Table: Comparison of Humoral and Cellular Immunity

Feature	Humoral Immunity	Cellular Immunity
Main Cells	B lymphocytes	T lymphocytes
Effector Molecules	Antibodies	Cytokines, cytotoxic granules
Targets	Extracellular pathogens	Intracellular pathogens, tumor cells
Memory	Memory B cells	Memory T cells

• Monoclonal antibodies are used in diagnostic tests and cancer therapy.

• HIV infection targets CD4+ T cells, impairing both humoral and cellular immunity.

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and tables summarizing immunoglobulin classes and T cell functions.