Chapter 16: Adaptive Immunity – Structure and Function in Microbiology

Adaptive Immunity

Introduction

Adaptive immunity is the body's highly specific defense mechanism against distinct pathogens and their products. Unlike innate immunity, adaptive immunity can recognize and remember specific antigens, leading to a more effective response upon subsequent exposures.

Objectives of Chapter 16

• Describe the components of adaptive immunity.

• Describe cell-mediated immunity.

• Describe humoral immunity.

Innate Immunity vs Adaptive Immunity

Innate and adaptive immunity are two branches of the immune system with distinct characteristics.

Feature	Innate Immunity	Adaptive Immunity
Distribution	Almost all multicellular eukaryotes	Only in vertebrates
Targets	Limited number of key structures (PAMPs), ~20-50	Antigens, mostly proteins; billions of types
Immune Receptors	Pattern recognition receptors (e.g., TLRs)	T cell receptors and antibodies
Cellular Presence	Almost all cells	Lymphocytes only
Discrimination	Host cells do not contain PAMPs	Tolerance for self-antigens; breakdown leads to autoimmunity

Overview of Adaptive Immunity

• The body recognizes antigens (foreign molecules).

• It produces antibodies or cytotoxic T cells (Tc cells) in response.

• These effectors destroy or deactivate the antigens.

Attributes of Adaptive Immunity

• Specificity: Targets unique antigens.

• Inducibility: Activated only in response to specific antigens.

• Clonality: Activated lymphocytes proliferate to form clones.

• Unresponsiveness to Self: Does not attack the body's own cells.

• Memory: Remembers antigens for faster, stronger responses upon re-exposure.

Example: B cells produce antibodies specific to SARS-CoV-2 after infection or vaccination, and memory B cells enable a rapid response to future exposures.

Main Components of the Immune Response

• Cell-mediated immunity: Involves T cells targeting infected or abnormal cells.

• Humoral (antibody-mediated) immunity: Involves B cells producing antibodies against extracellular pathogens.

Lymphocytes: The Key Cells of Adaptive Immunity

B Lymphocytes (B Cells)

• Located in spleen, lymph nodes, and mucosa-associated lymphoid tissue (MALT).

• Major function: Secretion of antibodies.

• Each B cell expresses a unique B cell receptor (BCR).

T Lymphocytes (T Cells)

• Produced in red bone marrow, mature in thymus.

• Migrate to lymph nodes, spleen, and Peyer's patches.

• Express T cell receptors (TCRs) on their surface.

Types of T Lymphocytes

• Cytotoxic T lymphocytes (Tc or CD8 cells): Directly kill infected or abnormal cells.

• Helper T lymphocytes (Th or CD4 cells): Regulate activities of B cells and Tc cells.

• Regulatory T lymphocytes (Tr cells): Suppress immune responses to prevent autoimmunity.

Lymphocyte	Site of Maturation	Representative Cell-Surface Glycoproteins	Notable Secretions
Helper T cell type 1 (Th1)	Thymus	CD4, distinctive TCR	Interleukin-2, IFN-γ
Helper T cell type 2 (Th2)	Thymus	CD4, distinctive TCR	Interleukin-4 and 5
Cytotoxic T cell (Tc)	Thymus	CD8, CD95, distinctive TCR	Perforin, granzyme
Regulatory T cell (Tr)	Thymus	CD4, CD25, distinctive TCR	Interleukin-10

Subpopulations of Helper T Cells

• Th1 cells: Assist cytotoxic T cells and activate macrophages.

• Th2 cells: Function with B cells to stimulate antibody production.

Helper T cells secrete cytokines that regulate the immune response.

Tissues and Organs of the Lymphatic System

• Includes lymphatic vessels, cells, tissues, and organs (e.g., lymph nodes, spleen, MALT).

• Function: Screen body tissues for foreign antigens.

Antigens

• Any substance that stimulates an immune response when introduced into the body.

• Common types: Proteins, glycoproteins, phospholipids.

• Contain epitopes (antigenic determinants) – regions recognized by immune cells.

Example: The spike protein of SARS-CoV-2 acts as an antigen, stimulating antibody production.

Preparation for an Adaptive Immune Response

Major Histocompatibility Complex (MHC)

• Cell surface glycoproteins encoded by genes on human chromosome 6.

• Function: Present antigenic peptides to T cells.

Types of MHC

• Class I MHC: Present on all nucleated cells (except RBCs); interact with CD8 (Tc) cells.

• Class II MHC: Present on antigen-presenting cells (APCs) such as B cells and macrophages; interact with CD4 (Th) cells.

Antigen Presentation

• Antigens are processed and displayed by MHC proteins on cell surfaces.

• Endogenous antigens: Presented by MHC I (e.g., viral proteins in infected cells).

• Exogenous antigens: Presented by MHC II (e.g., bacterial proteins taken up by phagocytosis).

Antibodies (Immunoglobulins)

• Soluble, protein molecules that bind antigens specifically.

• Secreted by plasma cells (differentiated B cells).

• Each antibody is monospecific (binds one epitope) and bivalent (has two binding sites).

Structure of Antibody

• Composed of two heavy chains and two light chains.

• Variable regions form antigen-binding sites.

• Constant region determines antibody class and function.

Functions of Antibodies

• Neutralization: Block attachment of toxins or pathogens to host cells.

• Opsonization: Enhance phagocytosis by marking pathogens for ingestion.

• Agglutination: Clump pathogens together, hindering their activity and facilitating clearance.

• Antibody-dependent cellular cytotoxicity (ADCC): Recruit NK cells to destroy antibody-coated cells.

Classes of Antibodies

Class	Structure	Main Function	Location	Half-life (days)
IgG	Monomer	Main serum antibody; crosses placenta; opsonization, neutralization	Serum, extravascular spaces	23
IgM	Pentamer	First antibody produced; agglutination; complement activation	Serum	5
IgA	Dimer (secretory)	Mucosal immunity; found in secretions (tears, saliva, mucus)	Secretions, mucosal surfaces	6
IgE	Monomer	Allergic responses; defense against parasites	Bound to mast cells, basophils	2
IgD	Monomer	B cell receptor (BCR) function	B cell surface	3

Cell-Mediated Immune Response

• Targets intracellular pathogens (e.g., viruses), abnormal cells (e.g., cancer), and some bacteria/protozoa.

• Involves activation of cytotoxic T cells (Tc) and helper T cells (Th).

Steps in Cell-Mediated Immunity

1. Antigen is processed and bound to MHC on antigen-presenting cells (APCs).

2. Th and Tc cells recognize the MHC-antigen complex.

3. APC produces IL-12, stimulating Th differentiation to Th1.

4. Th1 cells produce IL-2, activating Tc cells to divide into active and memory Tc cells.

5. IL-2 stimulates further division of active Tc cells.

Action of Cytotoxic T (Tc) Cells

• Perforin-granzyme pathway: Tc cells release perforin (forms pores) and granzyme (induces apoptosis) to kill target cells.

• CD95 pathway: Involves interaction with glycoproteins on target cells, leading to apoptosis.

Humoral (Antibody-Mediated) Immune Response

• Targets exogenous pathogens and toxins.

• Activated by specific antigens; involves B cells and helper T cells (Th2).

Steps in Humoral Immunity

1. Th cell recognizes MHC-antigen complex on APC and is activated.

2. IL-4 stimulates Th differentiation to Th2.

3. Th2 recognizes MHC-antigen complex on B cells.

4. Th2 produces IL-4, activating B cells.

5. Activated B cells differentiate into plasma cells (secrete antibodies) and memory B cells.

Plasma Cells and Memory B Cells

• Plasma cells: Short-lived, secrete large amounts of antibodies.

• Memory B cells: Long-lived, initiate rapid antibody production upon re-exposure to antigen.

Immunological Memory

• Primary response: First exposure to antigen; slower, produces fewer antibodies.

• Secondary response: Subsequent exposures; faster and stronger due to memory cells.

Types of Acquired Immunity

• Naturally acquired: Immune response to antigens encountered in daily life.

• Artificially acquired: Immune response to antigens introduced via vaccination.

• Active immunity: Body produces its own antibodies or T cells.

• Passive immunity: Antibodies are received from another individual (e.g., maternal antibodies).

Summary Table: Types of Acquired Immunity

Type	How Acquired	Example
Naturally acquired active	Infection/exposure to pathogen	Recovery from measles
Naturally acquired passive	Maternal antibodies	IgG crossing placenta
Artificially acquired active	Vaccination	MMR vaccine
Artificially acquired passive	Injection of antibodies	Antivenom for snakebite

Additional info: The notes include references to COVID-19 and SARS-CoV-2 as real-world examples of adaptive immunity, especially in the context of vaccination and antibody production.