Adaptive Immunity: Structure, Function, and Mechanisms

Chapter 17: Adaptive Immunity

Introduction to Adaptive Immunity

Adaptive immunity is a specific defense mechanism that develops in response to exposure to antigens. It involves highly specialized cells and processes that provide long-lasting protection and immunological memory.

Cytokines in Adaptive Immunity

Roles of Cytokines

• Cytokines are small proteins released by cells, especially immune cells, that regulate immunity, inflammation, and hematopoiesis.

• They act as chemical messengers, coordinating the immune response by promoting cell activation, differentiation, and migration.

• Examples: Interleukins (ILs), interferons (IFNs), tumor necrosis factors (TNFs).

Cytokine Storm

• A cytokine storm is an excessive and uncontrolled release of cytokines.

• This can lead to severe inflammation, tissue damage, and organ failure.

• Seen in severe infections (e.g., influenza, COVID-19).

Antigens, Epitopes, and Haptens

Antigen

• An antigen is any substance that can provoke an immune response by binding to antibodies or T cell receptors.

Epitope

• An epitope (antigenic determinant) is the specific part of an antigen recognized by the immune system.

Hapten

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

Anatomy of an Antibody (Immunoglobulin)

Structure of Antibodies

• Composed of four polypeptide chains: two heavy chains and two light chains.

• Each chain has a variable region (binds antigen) and a constant region (determines class and function).

• Valence: Number of antigen-binding sites (usually two per antibody molecule).

Classes of Immunoglobulins

Overview

There are five main classes of immunoglobulins, each with distinct structure and function.

Class	Structure	Function	Location
IgG	Monomer	Main antibody in serum; crosses placenta; neutralizes toxins	Blood, lymph, intestine
IgM	Pentamer	First antibody produced; effective in agglutination	Blood, lymph, B cell surface
IgA	Dimer (mainly)	Protects mucosal surfaces	Secretions (tears, saliva, mucus, breast milk)
IgD	Monomer	Functions as B cell receptor	B cell surface, blood
IgE	Monomer	Involved in allergic reactions; defense against parasites	Bound to mast cells, basophils

ABO Blood Typing and IgM

• IgM antibodies are responsible for agglutination reactions in ABO blood typing.

B Cells: Development and Function

Life of a B Cell

• Origin: Produced in the bone marrow.

• Maturation: Mature in the bone marrow; undergo clonal deletion to remove self-reactive cells.

• Migration: Move to lymphoid tissues (e.g., lymph nodes, spleen) to encounter antigens.

• Activation: Activated by binding to specific antigen and receiving help from T cells (for T-dependent antigens).

• Clonal Selection: Only B cells with receptors specific to the antigen proliferate and differentiate.

• T Cell Help: T helper cells provide confirmation signals (cytokines) for full activation.

• T-dependent vs T-independent Antigens: T-dependent require T cell help; T-independent can activate B cells directly (often polysaccharides).

Plasma Cells and Antibody Production

• Plasma cells are differentiated B cells that secrete large amounts of antibodies.

• Antibody functions include:

  • Agglutination: Clumping of antigens for easier removal.

  • Opsonization: Coating pathogens to enhance phagocytosis.

  • Antibody-dependent cell-mediated cytotoxicity (ADCC): Targeting cells for destruction by immune cells.

  • Neutralization: Blocking pathogen attachment or toxin activity.

  • Activation of complement: Triggering the complement cascade for pathogen lysis.

Memory Cells

• Memory B cells persist after infection, enabling a faster and stronger response upon re-exposure to the same antigen.

T Cells: Development and Function

Life of a T Cell

• Origin: Produced in the bone marrow.

• Maturation: Mature in the thymus; undergo thymic selection to eliminate self-reactive cells.

• Migration: Move to lymphoid tissues to encounter antigens presented by antigen-presenting cells (APCs).

T Cell Activation

• Activation requires antigen presentation via major histocompatibility complexes (MHC):

  • MHC Class I: Present on all nucleated cells; present endogenous antigens to CD8+ cytotoxic T cells; mark cells as "self".

  • MHC Class II: Present on APCs (dendritic cells, macrophages, B cells); present exogenous antigens to CD4+ helper T cells.

• Antigen Presenting Cells (APCs): Dendritic cells and macrophages process and present antigens to T cells, initiating the adaptive response.

• Clonal Selection: Only T cells with receptors specific to the presented antigen proliferate and differentiate.

Memory T Cells

• Memory T cells provide long-term immunity by responding rapidly to previously encountered antigens.

Classes of T Cells and Their Functions

• T Helper Cells (CD4+): Coordinate immune responses by secreting cytokines.

  • TH1: Activate macrophages and cytotoxic T cells; important in cell-mediated immunity.

  • TH2: Stimulate B cells to produce antibodies; important in humoral immunity.

  • TH17: Recruit neutrophils; important in defense against extracellular pathogens.

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

• Cytotoxic T Lymphocytes (CD8+): Destroy infected or abnormal cells by inducing apoptosis (programmed cell death).

Apoptosis

• Controlled cell death that prevents the spread of infection and minimizes inflammation.

Natural Killer (NK) Cells

• Natural killer cells are lymphocytes that can destroy virus-infected and tumor cells without prior sensitization.

• They recognize cells lacking MHC I and induce apoptosis.

Immunity: Primary and Secondary Responses

• Primary immune response: Occurs upon first exposure to an antigen; slower and less robust.

• Secondary immune response: Occurs upon subsequent exposures; faster and stronger due to memory cells.

• Cells involved: B cells, T cells, memory cells, plasma cells.

Types of Adaptive Immunity

Type	How Acquired	Example
Naturally acquired active	Infection/exposure to pathogen	Recovery from measles
Naturally acquired passive	Transfer of antibodies from mother	Maternal antibodies via placenta or breast milk
Artificially acquired active	Vaccination	MMR vaccine
Artificially acquired passive	Injection of antibodies	Antivenom, immunoglobulin therapy

Key Terms

• Antiserum: Blood serum containing antibodies against specific antigens; used for passive immunity.

• Serology: The study of reactions between antigens and antibodies in serum.

• Globulins: A group of serum proteins, including immunoglobulins (antibodies).

Additional info: Academic context and examples have been added to clarify and expand upon the original outline points.