Chapter 16: Adaptive Immunity – Microbiology Study Notes

Adaptive Immunity

Introduction to Adaptive Immunity

Adaptive immunity is the third line of defense in the immune system, providing specific protection against distinct pathogens. Unlike innate immunity, adaptive immunity is acquired and involves a highly specialized response to antigens.

• Specificity: Targets particular pathogens or foreign substances.

• Inducibility: Activated in response to specific antigens.

• Clonality: Generates clones of cells to fight the invader.

• Unresponsiveness to self: Prevents attack on the body's own cells.

• Memory: Remembers previous encounters for faster future responses.

Major players: B lymphocytes (plasma cells, antibodies) and T lymphocytes.

Lines of Immune Defense

Overview of Immune Defenses

• 1st Line (Nonspecific): Skin, mucous membranes, chemicals.

• 2nd Line (Nonspecific): Phagocytosis, complement, interferon, inflammation, fever.

• 3rd Line (Specific): Lymphocytes, antibodies (adaptive immunity).

The Lymphatic System

Role in Immunity

The lymphatic system is a network of vessels, nodes, and organs that transport lymph and house immune cells. It is essential for the development and deployment of adaptive immune responses.

• Lymph nodes: Filter lymph and provide sites for immune cell activation.

• Organs: Spleen, thymus, bone marrow, Peyer's patches.

Antigens and Epitopes

Definition and Examples

Antigens are molecules recognized as foreign by the immune system, triggering an immune response. Epitopes are specific 3-D regions on antigens that are recognized by immune cells.

• Bacterial components: Cell walls, capsules, pili, fimbriae, flagella.

• Viruses: Proteins and glycoproteins.

• Fungi/Protozoans: Cell walls, proteins.

• Other: Food allergens, dust, pollen.

Routes of entry: Ingestion, inhalation, breaks in skin/mucous membranes, direct injection, organ transplants, skin grafts.

MHC and Antigen Presenting Cells (APC)

Major Histocompatibility Complex (MHC)

MHC molecules are membrane glycoproteins that present antigens to T cells. They are crucial for distinguishing self from non-self and for activating adaptive immune responses.

• Class I MHC: Present on all nucleated cells (except RBCs); present endogenous antigens.

• Class II MHC: Present on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells; present exogenous antigens.

Antigen Processing

Exogenous Antigen Processing

Exogenous antigens (from outside the cell, e.g., bacteria, toxins) are processed by APCs and displayed on Class II MHC molecules.

• Phagocytosis by macrophages/B cells/dendritic cells.

• Digestion in phagolysosome.

• Presentation on MHC II.

Endogenous Antigen Processing

Endogenous antigens (from inside the cell, e.g., viral or tumor antigens) are processed and displayed on Class I MHC molecules.

• Processed in endoplasmic reticulum.

• Presented on MHC I.

Processing Type	Source	Processing Site	MHC Class
Exogenous	Extracellular bacteria/viruses	Phagolysosome	MHC II
Endogenous	Intracellular bacteria, viral/cancer proteins	Endoplasmic reticulum	MHC I

T Lymphocytes in Adaptive Immunity

Cell-Mediated Immune Response

T lymphocytes are produced in the bone marrow and mature in the thymus. They circulate in lymph and blood, targeting cells harboring intracellular pathogens and cancer cells.

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

• Helper T cells (Th, CD4): Regulate B cells and cytotoxic T cells; include Th1 (assist Tc) and Th2 (assist B cells).

• Regulatory T cells (Tr): Suppress immune responses to maintain tolerance.

T cell receptors (TCRs): Recognize specific epitope-MHC complexes; each T cell has a unique TCR.

Clonal Deletion

Self-Tolerance Mechanism

Clonal deletion eliminates self-reactive T lymphocytes via apoptosis, preventing autoimmune responses and ensuring self-tolerance.

Activation and Proliferation of Cytotoxic T Cells

Steps in Activation

• Antigen presentation by APCs.

• Helper T cell differentiation.

• Clonal expansion of cytotoxic T cells.

• Self-stimulation for further proliferation.

Memory T Cells

Long-Term Immunity

Some activated T cells become memory T cells, persisting in lymphoid tissues and providing rapid, effective responses upon re-exposure to the same antigen.

Cytotoxic T Cell Killing Mechanisms

Pathways

• Perforin-granzyme pathway: Synthesis of proteins that induce apoptosis in target cells.

• CD95 pathway: Mediated by glycoproteins on body cells, leading to apoptosis.

Antibody Immune Responses

Types of Antibody Immunity

Antibody responses are mounted against exogenous pathogens and toxins, and are activated only in response to specific antigens.

• T-Independent Antibody Immunity: Induced by large, repetitive antigens; does not require helper T cells; weak and short-lived, with little memory.

• T-Dependent Antibody Immunity: Induced by small, non-repetitive antigens; requires helper T cells; produces strong, long-lasting immunity with memory.

B Lymphocytes (B Cells) and Antibodies

Antibody Classes and Functions

B cells mature in bone marrow and are responsible for producing antibodies (immunoglobulins) specific to antigens. Different infections require different antibodies, depending on the antigen and site of infection.

• IgM: First antibody produced during an immune response.

• IgG: Most common and longest-lasting antibody.

• IgA: Associated with body secretions (e.g., saliva, mucus).

• IgE: Involved in responses to parasitic infections and allergies.

• IgD: Function not well understood.

B Cell Receptor (BCR) Specificity

• Each B cell has multiple copies of a unique BCR.

• BCRs recognize specific epitopes; the repertoire allows recognition of millions of different antigens.

Clonal Deletion of B Cells

Self-Tolerance in B Cells

Self-reactive B cells are deleted or inactivated in the bone marrow, similar to T cells, to prevent autoimmune responses.

Inducement of Antibody Immunity

T-Independent vs. T-Dependent Responses

• T-Independent: Large, repetitive antigens directly stimulate B cells; weak, short-lived, little memory.

• T-Dependent: Small, non-repetitive antigens require helper T cells; strong, long-lasting, induces memory.

Memory B Cells

Role in Secondary Immune Response

• Produced during B cell proliferation; do not secrete antibodies immediately.

• Persist in lymphoid tissue; initiate rapid antibody production upon re-exposure to antigen.

Types of Acquired Immunity

Natural vs. Artificial

• Naturally acquired: Response to antigens encountered in daily life.

• Artificially acquired: Response to antigens introduced via vaccination.

Cytokines in Immune Response

Types and Functions

• Interleukins (ILs): Signal among leukocytes.

• Interferons (IFNs): Antiviral proteins that may act as cytokines.

• Growth factors: Stimulate stem cell division.

• Tumor necrosis factor (TNF): Secreted by macrophages and T cells to kill tumor cells and regulate immune responses/inflammation.

• Chemokines: Chemotactic cytokines that signal leukocytes to move.

Summary Table: Key Cells and Functions in Adaptive Immunity

Cell Type	Function	Maturation Site
B lymphocyte	Produces antibodies	Bone marrow
Plasma cell	Secretes antibodies	Derived from B cell
T lymphocyte	Cell-mediated immunity	Thymus
Cytotoxic T cell (Tc, CD8)	Kills infected/cancer cells	Thymus
Helper T cell (Th, CD4)	Regulates B and Tc cells	Thymus
Regulatory T cell (Tr)	Suppresses immune response	Thymus

Key Equations and Concepts

• Clonal Expansion: $N = N_0 \times 2^n$ (where $N$ is the number of cells after $n$ divisions, $N_0$ is the initial number of cells)

• Antibody-Antigen Binding: $\text{Antibody} + \text{Antigen} \rightarrow \text{Immune Complex}$

Additional info: These notes synthesize both textual and visual content from lecture slides and textbook images, providing a comprehensive overview of adaptive immunity for microbiology students.