Adaptive Immune System: Body Defenses (Chapter 21) – Study Notes

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Immune System: Overview

Introduction to Body Defenses

The immune system protects the body from disease by identifying and eliminating pathogens and abnormal cells. It is divided into two main defense systems: innate (nonspecific) and adaptive (specific) immunity.

Innate (Nonspecific) Immunity: Provides immediate, general defense against pathogens. Includes physical barriers (skin, mucous membranes), phagocytic cells, antimicrobial proteins, and inflammation.
Adaptive (Specific) Immunity: Targets specific pathogens with a slower but more precise response. Involves lymphocytes (B and T cells) and develops immunological memory.

Adaptive (Specific) Immune System

Key Characteristics

Specificity: Recognizes and targets specific antigens.
Systemic Response: Not restricted to the initial infection site.
Memory: Mounts stronger attacks upon re-exposure to the same antigen.

Major Components

Lymphocytes: White blood cells responsible for adaptive immunity. Two main types: B cells (humoral immunity) and T cells (cell-mediated immunity).
Antigen-Presenting Cells (APCs): Cells such as dendritic cells, macrophages, and B cells that process and present antigens to lymphocytes.

Antigens and Immunological Recognition

Definition and Properties of Antigens

Antigen: Any substance that can provoke an immune response by being recognized as foreign (nonself).
Immunogenicity: The ability to stimulate proliferation of specific lymphocytes.
Reactivity: The ability to react with activated lymphocytes and antibodies.
Complete Antigens: Have both immunogenicity and reactivity (e.g., proteins, polysaccharides).
Haptens (Incomplete Antigens): Small molecules that are only immunogenic when attached to body proteins.

Antigenic Determinants

Specific regions on antigens (epitopes) that antibodies or lymphocyte receptors bind to.
Most antigens have multiple determinants, allowing activation of different lymphocyte populations.

Self-Antigens and MHC Proteins

Self-Antigen: Molecules on the surface of body cells that are not normally recognized as foreign.
Major Histocompatibility Complex (MHC) Proteins: Glycoproteins on cell surfaces that help the immune system distinguish self from nonself.
MHC Class I: Present on all nucleated cells; display endogenous antigens.
MHC Class II: Present on APCs; display exogenous antigens.

Lymphocyte Development and Maturation

Immunocompetence and Self-Tolerance

Immunocompetence: The ability of lymphocytes to recognize a specific antigen by binding to it.
Self-Tolerance: The ability to remain unresponsive to self-antigens.
B cells mature in the bone marrow; T cells mature in the thymus.

Clonal Selection and Memory

When a lymphocyte encounters its specific antigen, it is activated and proliferates, forming a clone of identical cells.
Some cells become effector cells (actively fight infection), others become memory cells (provide rapid response upon re-exposure).

Humoral (Antibody-Mediated) Immunity

B Cell Activation and Antibody Production

B cells are activated when antigens bind to their surface receptors, leading to clonal selection.
Most activated B cells become plasma cells that secrete antibodies; some become memory B cells.
Antibodies circulate in blood and lymph, binding to antigens and marking them for destruction.

Primary and Secondary Immune Responses

Primary Response: First exposure to antigen; slow, with antibody levels peaking in about 10 days.
Secondary Response: Subsequent exposure; faster and stronger due to memory cells, with antibody levels peaking in 2-3 days.

Active vs. Passive Humoral Immunity

Type	Acquisition	Immunological Memory	Examples
Active (Natural)	Infection; contact with pathogen	Yes	Chickenpox infection
Active (Artificial)	Vaccine; dead or attenuated pathogens	Yes	MMR vaccine
Passive (Natural)	Antibodies passed from mother to fetus via placenta or to infant via milk	No	Maternal antibodies
Passive (Artificial)	Injection of exogenous antibodies (gamma globulin)	No	Antivenom

Antibodies (Immunoglobulins)

Structure and Classes

Y-shaped proteins composed of two heavy and two light chains, with variable regions forming antigen-binding sites.
Five main classes: IgM, IgA, IgD, IgG, IgE.

Class	Main Features	Functions
IgM	Pentamer; first antibody released	Potent agglutinating agent; activates complement
IgA	Dimer; in secretions (mucus, saliva)	Prevents pathogen entry
IgD	Monomer; on B cell surface	B cell receptor
IgG	Monomer; most abundant in plasma	Main antibody of secondary response; crosses placenta
IgE	Monomer; in plasma, on mast cells and basophils	Allergic reactions; defense against parasites

Mechanisms of Antibody Action

Neutralization: Antibodies block pathogen binding sites.
Agglutination: Antibodies cause antigens to clump together.
Precipitation: Soluble antigens are cross-linked and precipitate out of solution.
Complement Activation: Antibody-antigen complexes activate complement proteins, leading to cell lysis.

Cell-Mediated Immunity

T Cell Types and Functions

Helper T Cells (CD4+): Activate B cells, cytotoxic T cells, and macrophages; direct adaptive immune response.
Cytotoxic T Cells (CD8+): Directly kill infected or abnormal cells.
Regulatory T Cells: Suppress immune response to prevent autoimmunity.
Memory T Cells: Provide rapid response upon re-exposure to antigen.

Antigen Presentation and MHC

T cells recognize antigens only when presented on MHC proteins by APCs.
MHC Class I: Present endogenous antigens to cytotoxic T cells.
MHC Class II: Present exogenous antigens to helper T cells.

T Cell Activation and Clonal Expansion

Requires antigen binding and co-stimulatory signals from APCs.
Activated T cells proliferate and differentiate into effector and memory cells.

Immunological Disorders

Immunodeficiencies

Primary (Congenital): Genetic defects (e.g., SCID) causing deficits in B and T cells.
Acquired: Result from infections (e.g., HIV/AIDS) or treatments that suppress the immune system.

Autoimmune Diseases

Immune system fails to distinguish self from nonself, attacking body tissues.
Examples: Rheumatoid arthritis, multiple sclerosis, type 1 diabetes mellitus, systemic lupus erythematosus (SLE).

Hypersensitivities

Type I (Immediate, Allergies): Rapid IgE-mediated response to allergens (e.g., pollen, food).
Type II (Cytotoxic): Antibody-mediated destruction of cells (e.g., mismatched blood transfusion).
Type III (Immune Complex): Antigen-antibody complexes cause inflammation (e.g., SLE).
Type IV (Delayed): T cell-mediated response (e.g., contact dermatitis, poison ivy).

Clinical Applications

Vaccines and Antibiotics

Vaccines: Provide artificial active immunity by exposing the immune system to harmless forms of pathogens.
Antibiotics: Drugs that kill or inhibit the growth of bacteria; do not provide immunity but help control infections.

Monoclonal Antibodies and Diagnostic Tools

Monoclonal Antibodies: Laboratory-produced antibodies used in research, diagnostics, and therapy.
ELISA (Enzyme-Linked Immunosorbent Assay): A test that uses antibodies and color change to identify a substance.

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Nonspecific	Specific
Response Time	Immediate	Slower (days)
Memory	No	Yes
Cells Involved	Phagocytes, NK cells	B and T lymphocytes
Main Components	Barriers, inflammation	Antibodies, cytotoxic cells

Additional info:

Some content was inferred and expanded for clarity and completeness, such as the structure and function of antibodies, and the summary tables.
For equations: The immune system does not typically use mathematical equations, but if needed, the following is a general representation of antibody-antigen binding: