Adaptive Immunity: Specific Defenses of the Host (Chapter 17) Study Guide

Adaptive Immunity: Specific Defenses of the Host

Comparison of Adaptive and Innate Immunity

Immunity is the body's ability to defend against pathogens. It is divided into innate and adaptive immunity, each with distinct characteristics.

• Innate Immunity: Present from birth, non-specific, rapid response, includes barriers (skin, mucous membranes), phagocytes, inflammation, and complement proteins.

• Adaptive Immunity: Acquired after exposure to antigens, specific to pathogens, slower initial response, involves lymphocytes (B and T cells), memory formation for faster future responses.

Humoral vs. Cellular Immunity

Adaptive immunity is further divided into humoral and cellular immunity.

• Humoral Immunity: Mediated by B cells and antibodies; targets extracellular pathogens.

• Cellular Immunity: Mediated by T cells; targets intracellular pathogens (e.g., viruses, some bacteria).

Functions of Cytokines and Related Molecules

Cytokines are signaling proteins that regulate immune responses.

• Cytokines: General term for immune signaling molecules; coordinate cell communication.

• Interleukins (IL): Mediate communication between leukocytes; regulate growth and differentiation.

• Chemokines: Attract immune cells to sites of infection (chemotaxis).

• Interferons (IFN): Inhibit viral replication; activate immune cells.

• Tumor Necrosis Factor (TNF): Promotes inflammation; can induce apoptosis.

• Hematopoietic Cytokines: Stimulate formation of blood cells.

Antigens, Epitopes, and Haptens

Antigens are substances that provoke an immune response.

• Antigen: Any molecule recognized by the immune system as foreign.

• Epitope: Specific region of an antigen recognized by antibodies or T cells.

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

Antibody Structure and Function

Antibodies (immunoglobulins) are proteins produced by B cells that bind antigens.

• Structure: Y-shaped molecule with two identical heavy and two identical light chains; variable region binds antigen.

• Function: Neutralize pathogens, opsonize for phagocytosis, activate complement.

Chemical Characteristics: Antibodies are glycoproteins with variable (V) and constant (C) regions.

Classes of Antibodies and Their Functions

There are five main classes of antibodies, each with unique roles.

Class	Main Function
IgG	Most abundant; crosses placenta; neutralizes toxins
IgM	First produced in response; activates complement
IgA	Found in mucosal secretions; protects mucosal surfaces
IgD	Acts as B cell receptor
IgE	Involved in allergic reactions; defends against parasites

T-Dependent vs. T-Independent Antigens

• T-Dependent Antigens: Require T helper cells for B cell activation; usually proteins.

• T-Independent Antigens: Can activate B cells directly; usually polysaccharides; produce weaker, short-lived responses.

Plasmocyte (Plasma Cell) vs. Memory Cell

• Plasma Cell: Differentiated B cell that produces antibodies.

• Memory Cell: Long-lived B or T cell that responds rapidly upon re-exposure to antigen.

Clonal Selection

Clonal selection is the process by which specific lymphocytes are activated and proliferate in response to an antigen.

• Only lymphocytes with receptors specific to the antigen are selected.

• These cells proliferate and differentiate into effector and memory cells.

Immunoglobulin Variable Region

The variable region of the immunoglobulin molecule has an amino acid sequence that differs from B cell to B cell, allowing for antibody diversity.

Outcomes of Antigen–Antibody Reactions

• Neutralization: Antibodies block pathogen attachment or toxin activity.

• Opsonization: Antibodies coat pathogens, enhancing phagocytosis.

• Agglutination: Antibodies cause pathogens to clump together.

• Complement Activation: Antibody binding triggers complement cascade, leading to cell lysis.

Functions of Immune Cells

• M Cells: Facilitate antigen sampling in mucosal tissues.

• TH Cells (T Helper): Activate B cells and other T cells; secrete cytokines.

• TC Cells (T Cytotoxic): Destroy infected or abnormal cells.

• Treg Cells (T Regulatory): Suppress immune responses to prevent autoimmunity.

• NK Cells (Natural Killer): Kill virus-infected and tumor cells without prior sensitization.

Antigen-Presenting Cells (APCs)

APCs are cells that process and present antigens to T cells, initiating adaptive immune responses.

• Examples: Dendritic cells, macrophages, B cells.

T Helper, T Cytotoxic, and T Regulatory Cells

• T Helper (TH): Coordinate immune responses via cytokine secretion.

• T Cytotoxic (TC): Directly kill infected cells.

• T Regulatory (Treg): Modulate immune response, prevent excessive reactions.

TH1, TH2, and TH17 Cells

• TH1: Promote cellular immunity; activate macrophages and TC cells.

• TH2: Promote humoral immunity; stimulate B cell antibody production.

• TH17: Recruit neutrophils; important in inflammation and defense against fungi and bacteria.

Apoptosis

Apoptosis is programmed cell death, a controlled process to eliminate damaged or unnecessary cells without causing inflammation.

Function of Natural Killer Cells

NK cells recognize and kill cells lacking "self" markers (MHC I), such as virus-infected or tumor cells.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

ADCC is a process where antibodies bind to target cells, and NK cells recognize these antibodies, leading to cell destruction.

Primary vs. Secondary Immune Response

• Primary Response: First exposure to antigen; slower, lower antibody levels.

• Secondary Response: Subsequent exposure; faster, higher antibody levels due to memory cells.

Types of Adaptive Immunity

Type	Source	Example
Natural Active	Exposure to pathogen	Recovery from infection
Natural Passive	Transfer from mother	Maternal antibodies
Artificial Active	Vaccination	Immunization
Artificial Passive	Injection of antibodies	Antiserum therapy