Adaptive Immunity: The Role of B Cells and Antibodies

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Adaptive Immunity: The Role of B Cells and Antibodies

Overview of the Immune System

The immune system is divided into two main branches: innate immunity and adaptive immunity. Innate immunity is present at birth, always active, non-specific, and rapid. Adaptive immunity is pathogen-specific, develops memory, is activated upon infection, and responds more rapidly upon re-exposure.

Innate Immunity: Non-specific, immediate defense mechanisms present from birth.
Adaptive Immunity: Specific to particular pathogens, develops over time, and provides long-lasting protection through memory cells.

Branches of Adaptive Immunity

Humoral (Antibody-Mediated) Immunity: B cell-based; involves the production of antibodies that circulate in body fluids.
Cell-Mediated Immunity: T cell-based; involves the activation of T lymphocytes to destroy infected or abnormal cells.

B Lymphocytes (B Cells)

Development and Location

B cells are born and mature in the red bone marrow.
They are found primarily in the spleen, lymph nodes, and MALT (mucosa-associated lymphoid tissue).
A small percentage of B cells circulate in the blood.
The major function of B cells is the secretion of antibodies.

B Cell Receptors (BCRs)

Each B lymphocyte has multiple identical copies of the B cell receptor (BCR) on its surface, which are unique to that cell.

Each B cell has about 500,000 BCRs on its surface.
BCRs are made up of six polypeptide chains: two identical heavy chains and two identical light chains forming a Y-shape, plus two additional chains attached to the stem.
The ends of the arms of the Y are variable regions that form the antigen-binding sites.
Antigen-binding sites are complementary to the three-dimensional shape of an epitope (the specific part of an antigen recognized by the immune system).

Diversity of BCRs

Each person has between and B cells, each with unique BCRs, despite having only about 25,000 genes. This diversity is generated by genetic recombination of gene segments.

The variable region of the BCR is coded by DNA stretches called loci.
BCR genes occur at three loci, one on each chromosome.
Heavy chain locus: 65 variable segments, 27 diversity segments, 6 junction segments.
Combinatorial diversity: possible combinations per chromosome; chromosomes = .

Additional info: Further diversity is generated by junctional diversity and somatic hypermutation.

Antigens and Epitopes

Definition and Properties

Antigens are molecules recognized as foreign and capable of eliciting an immune response.
Recognized by three-dimensional regions called epitopes (antigenic determinants).
Large, complex macromolecules (proteins, polysaccharides) are the best antigens.
Sources include bacterial components, viral proteins, fungi, protozoa, and even food or dust particles.

Antibodies (Immunoglobulins)

Structure and Function

Antibodies are secreted by activated B cells called plasma cells.
They have identical antigen-binding sites and specificity as the BCR of the activated B cell.
Antigen-binding sites are complementary to epitopes.

Classes of Antibodies

There are five main classes of antibodies, each with distinct properties and functions:

Name	Properties	Structure
IgA	Found in mucous, saliva, tears, and breast milk. Protects against pathogens.	Dimer (two Y-shaped units joined)
IgD	Part of the B cell receptor. Activates basophils and mast cells.	Monomer (single Y-shaped unit)
IgE	Protects against parasitic worms. Responsible for allergic reactions.	Monomer
IgG	Secreted by plasma cells in the blood. Able to cross the placenta into the fetus. Most abundant and long-lasting.	Monomer
IgM	May be attached to the surface of a B cell or secreted into the blood. Responsible for early stages of immunity. First antibody produced.	Pentamer (five Y-shaped units joined)

Class Switching

Class switching is the process by which the variable region of an antibody is attached to a different constant (stem) region gene, allowing the antibody to change its class (e.g., from IgM to IgG) while retaining antigen specificity.

IgM: First antibody produced in response to an antigen.
IgG: Most common and long-lasting antibody; major antibody in the body.
IgA: Associated with body secretions.
IgE: Involved in responses to parasitic infections and allergies.
IgD: Activates basophils and mast cells.

Functions of Antibodies

Antibodies perform several key functions in the immune response:

Activation of complement and inflammation: Antibody binding can trigger the complement cascade, leading to pathogen lysis and inflammation.
Neutralization: Antibodies bind to toxins or pathogen surface proteins, preventing their harmful effects.
Opsonization: Antibodies coat pathogens, enhancing their uptake and destruction by phagocytes.
Killing by oxidation: Some antibodies can trigger the production of reactive oxygen species to kill pathogens.
Agglutination: Antibodies bind multiple pathogens together, causing them to clump and precipitate (notably IgG and IgA).
Antibody-dependent cellular cytotoxicity (ADCC): Antibodies recruit natural killer (NK) cells to destroy antibody-coated target cells.

Clonal Deletion of B Cells

Clonal deletion is a process that removes self-reactive B cells during development to prevent autoimmunity.

B cells that do not bind to self-antigens proceed to further maturation.
Mature T and B cells enter the blood and lymphoid tissues, where they scan for antigens.
Some B cells can change their BCRs instead of undergoing apoptosis.

Antibody Immune Response

T-Independent Antibody Immunity

Large antigens with repeating epitopes can activate B cells without T cell help (T-independent response).
These responses are less robust in children due to a less extensive B cell repertoire.

Plasma Cells

Plasma cells are activated B cells specialized for antibody secretion.
They are larger than normal B cells and have abundant rough endoplasmic reticulum and Golgi bodies to support high rates of antibody production.
Plasma cells are short-lived but produce large quantities of antibodies.

T-Dependent Antibody Immunity and Clonal Selection

B cells require help from T helper (Th) cells for activation in response to most antigens (T-dependent response).
Antigen-presenting cells (APCs) acquire antigens and present them to Th cells in lymph nodes.
Th cells differentiate into Th2 cells (mediated by IL-4) and activate B cells by binding to MHC II-antigen complexes on B cells.
Activated B cells proliferate and differentiate into plasma cells (antibody-secreting) and memory B cells.

Memory B Cells and Immune Memory

Memory B cells are produced during B cell proliferation but do not secrete antibodies.
They have BCRs complementary to the original antigen and persist long-term in lymphoid tissues.
Upon re-exposure to the antigen, memory B cells rapidly initiate antibody production, resulting in a faster and stronger secondary immune response compared to the primary response.

Summary Table: Key Features of B Cell-Mediated Immunity

Feature	Description
B Cell Diversity	Generated by recombination of gene segments; each person has billions of unique BCRs.
Antibody Classes	IgM, IgG, IgA, IgE, IgD; each with specific roles and structures.
Antibody Functions	Complement activation, neutralization, opsonization, agglutination, ADCC.
Plasma Cells	Short-lived, secrete large amounts of antibodies.
Memory B Cells	Long-lived, responsible for rapid secondary immune response.
Clonal Deletion	Eliminates self-reactive B cells to prevent autoimmunity.

Recap

B cells are central to adaptive (humoral) immunity.
Each person has a vast repertoire of B cells with unique receptors.
Antibodies are secreted BCRs with diverse functions.
Five classes of antibodies: IgM, IgG, IgA, IgE, IgD.
B cell activation often requires T helper cells.
B cells differentiate into plasma cells (short-lived, secrete antibodies) and memory cells (long-lived, rapid response).