Naïve B cells require help from CD4+ T helper cells for full activation.
They present antigens as peptides bound to MHC II, activating CD4+ helper T cells, which then stimulate the B cell.
Plasma cells, or activated B cells, secrete antibodies that perform various functions such as:
Neutralization
Opsonization
Complement activation
Memory B cells provide a long-lived response.
Antigen Specificity
Antigens are toxins or foreign substances that induce an immune response.
Specificity refers to the ability of an antibody to bind to one, but not another, member of a family of related substances.
B Cells and Antibodies
Each B cell is specific for one antigen, recognizing a specific region on a protein.
Antigens are recognized by immunoglobulin (Ig) molecules.
One B cell expresses one Ig sequence.
Surface-bound Ig on B cells is called the B cell receptor (BCR).
Soluble, secreted Ig is called an antibody.
Structure of an Immunoglobulin Molecule
The immunoglobulin molecule consists of:
An N terminus, which is the variable region that binds to the antigen.
A C terminus, which is the constant region that binds to Fc receptors and mediates Ig functions.
Two heavy chains and two light chains joined by disulfide bonds.
Antigen Binding Sites
Immunoglobulin molecules bind to regions on the protein, recognizing conformational shapes.
Diversity in Antigen Receptors
Diversity is generated through the combination of immunoglobulin gene segments:
Variable (V)
Diversity (D)
Junctional (J)
Constant (C)
During B cell development, these gene elements rearrange through VDJ recombination.
The diversity region is in contact with the antigen.
Different combinations of these genes enable diversity, with approximately 5 \times 10^{13} possible combinations.
B Cell Development
Lymphocytes develop immunocompetence in the primary lymphoid organs.
B cells mature in the bone marrow.
Rearrangement of Ig gene elements generates a B Cell Receptor (VDJ recombination).
Any self-reactive cell is deleted during development through negative selection.
Immature B cells leave the bone marrow ready to scan the body.
Negative Selection
The immune system must respond to foreign proteins but not self-proteins.
Self/non-self discrimination is crucial.
It is important to delete any B cells that are specific for a self-protein.
An array of self-antigens is expressed in the bone marrow to facilitate this process.
Activation of B Cell/Antibody Response
The B cell receptor (surface-bound immunoglobulin) binds to the antigen.
The B cell presents a peptide to antigen-specific CD4+ T helper cells via MHC II.
CD4+ T cells provide co-stimulation to the B cell.
B cell activation induces proliferation and differentiation into antibody-secreting plasma cells and memory cells.
Microbial constituents, such as bacterial polysaccharides, can stimulate B cells independently of T cells.
T Cells Activate B Cells
T cells activate B cells via membrane-bound and secreted molecules.
Membrane-bound: CD40 (co-stimulation) for B cell activation.
Soluble: Cytokines for B cell differentiation.
B & T Cell Activation
B and T cell activation occurs in secondary lymphoid organs, such as lymph nodes and spleens.
B and T cells interact at border zones within these organs (B-cell follicle and T cell zone).
B Cell Response Refinement in Germinal Center
Once activated, B cells migrate into follicles and form a germinal center where immunoglobulin modifications occur, supported by accessory immune cells.
Somatic hypermutation (Ig gene editing) takes place to improve antibody affinity (binding strength).
Class-switching occurs to generate different immunoglobulin isotypes (different functional roles).
Selection of high-affinity B cells ensures a faster and more effective response upon second exposure to the antigen.
Germinal Centers
Germinal centers are responsible for:
The generation of memory B cells.
The generation of high-affinity plasma cells.
Antibody-secreting plasma cells migrate to and reside in the bone marrow under homeostatic conditions.
Summary of B Cells
Each B cell expresses a unique immunoglobulin molecule, making it antigen-specific.
Immunoglobulin molecules are composed of a combination of Ig genes (VDJ and constant region).
B cells produce antibodies (secreted immunoglobulin molecules).
B cells are APCs that express MHC II.
B cells activate CD4+ T cells and receive co-stimulation.
They need help from CD4+ T cells for full activation.
Activation leads to the generation of plasma and memory cells.
B cells mediate the humoral immune response.
Antibody Functions
Antibodies perform a variety of functions:
Neutralize pathogens by binding to their surface and blocking entry into cells.
Opsonize pathogens by coating them with antibodies, targeting them for destruction by other immune cells (e.g., macrophages, NK cells, mast cells, and eosinophils).
Activate the complement pathway.
These functions are performed by the constant region on the immunoglobulin.
A variety of immunoglobulin isotypes perform these functions, with the same variable region but a different constant region.
Immunoglobulin Isotypes
Isotypes are determined by the constant region on the heavy chain:
IgA (IgA1, IgA2)
IgD
IgE
IgG (IgG1, IgG2a, IgG2b, IgG3, IgG4)
IgM
IgD and IgM can be expressed as surface-bound receptors on naïve B cells.
Somatic recombination (during B cell differentiation) leads to class switching and the production of different Ig isotypes.
Role of Cytokines
Cytokines induce switching to different isotypes.
Class Switching
Class switching leads to the activation of the heavy chain C-region gene.
Immunoglobulin Isotypes Trigger Different Responses
Functional activities of different isotypes include neutralization, opsonization, sensitization for killing by NK cells, sensitization of mast cells, and activation of the complement system.
The Humoral Immune Response
The humoral immune response involves:
B-cell activation by antigen and helper T cells.
Antibody secretion by plasma cells.
Neutralization, opsonization, and complement activation.
Neutralization: Bacterial/Parasitic Toxins
Pre-existing antibody is necessary.
It takes a while to initiate the adaptive immune responses, making it necessary to respond to toxins quickly.
Common Diseases Caused by Bacterial Toxins
Examples include tetanus (tetanus toxin), diphtheria (diphtheria toxin), gas gangrene (clostridial toxin), cholera (cholera toxin), anthrax (anthrax toxic complex), botulism (botulinum toxin), and others.
Neutralization: Viral Entry
Pre-existing antibody is necessary.
This is the basis for some vaccinations against viruses (e.g., COVID vaccines induce antibodies to spike protein).
Neutralization: Intracellular Bacterial Entry
This applies to atypical bacteria such as Chlamydia and mycoplasma species.
Antibodies prevent the attachment of bacteria to the cell surface.
Antibody-Dependent Cellular Cytotoxicity (ADCC)
IgG-coated target cells activate Fc receptors on NK cells, leading to the release of membrane-puncturing granules such as granzyme and perforin.
IgE-Mediated Degranulation
Mast cells and basophils bind IgE via high-affinity Fc epsilon receptors.
Antigen binds to mast cell-bound IgE, resulting in crosslinking of IgE and release of mediators by mast cells.
This is part of the anti-parasitic response and Type I Hypersensitivity (e.g., asthma and allergy).
Opsonization
Antibodies bound to the pathogen bind to receptors on phagocytic cells (macrophages), triggering phagocytosis and pathogen engulfment.
Opsonization Definition
Coating of a pathogen by antibodies and/or complement proteins to facilitate phagocytosis and destruction of the pathogen.
Opsonization Mechanism
Opsonization can occur through IgG:Fc binding or C3b:CR1 binding.
The bacterium is coated with complement and IgG antibody.
When C3b binds to CR1 and antibody binds to Fc receptor, bacteria are phagocytosed.
Macrophage membranes fuse, creating a membrane-enclosed vesicle, the phagosome.
Lysosomes fuse with these vesicles, delivering enzymes that degrade the bacteria.
Complement Activation
The complement system enhances the activation of phagocytes and pathogen uptake.
Complement System
The complement system is a component of the innate immune system that complements the action of antibodies.
It is a collection of plasma proteins (>30), many of which are pro-enzymes (zymogens) that are activated by proteolysis.
Activation causes a cascade of reactions leading to the destruction of pathogens.
It needs to be tightly regulated to prevent host damage.
Complement System: Three Pathways
The three pathways are:
The lectin pathway.
The classical pathway.
The alternative pathway.
All pathways converge on C3.
Complement System: Three Functions
Some complement proteins initiate inflammation (e.g., C3a, C5a).
Phagocytes have complement receptors (CR) and can bind "flagged" pathogens (i.e., opsonized).
C3b can bind to CR1.
Formation of the membrane-attack complex (MAC) can lyse certain pathogens.
Antibody:Antigen Binds to C1q
Complement activation is initiated when antibodies attached to the surface of a pathogen bind C1q.
C1q cannot bind IgM or IgG in serum.
IgM must undergo a conformational change to expose binding sites.
IgG molecules must be adjacent to create 2 binding sites and sufficient affinity for C1q.
Antibody:Antigen Complexes
Antibody:antigen complexes may have too few molecules of IgG to efficiently bind to Fc receptors.
Antigen can be coated with complement and transported to relevant organs (e.g., spleen) for destruction.
This can lead to insoluble immune complex deposition and complement activation, leading to chronic inflammation in autoimmune diseases and serum sickness (Type III hypersensitivity).
Complement receptors are important in the removal of immune complexes from the circulation.
Summary
B cells present antigen to CD4+ Helper T cells to ensure full activation.
This leads to proliferation and differentiation into plasma or memory B cells.
Cytokines released by CD4+ T cells promote isotype class switching.
Germinal center reactions ensure maturation of the B cell response, resulting in higher affinity antibodies and isotype class switching.
B cells secrete immunoglobulin molecules (antibodies) that mediate effector functions: