B Cells and Adaptive Immune Response

B Cells

• B cells are key components of the adaptive immune response.
• They play critical roles in antibody production and antigen presentation.
• B cells are central to the clonal selection theory.
• Clonal selection theory proposes antigen-specific B cells are activated and expanded upon encountering their cognate antigen.
• This theory is fundamental to understanding how the immune system responds to diverse pathogens.
• Antibody diversity is generated through processes including somatic hypermutation.
• This ensures a broad repertoire of antibodies capable of recognizing various antigens.

Adaptive Immune Response

• The adaptive response begins approximately 96 hours after initial infection.
• It is activated if the innate immune system fails to control the infection.
• B and T lymphocytes are the key players in the adaptive response.
• B cells mediate the adaptive humoral immune response.
• B cells produce antibodies (plasma cells).
• T cells mediate the adaptive cell-mediated immune response.
• T cells include Tc, Th1, Th2, TEH, and Th17 cells.
• Activation of the adaptive immune response occurs in secondary lymphoid tissue (e.g., lymph nodes, spleen, MALT).

Clonal Selection Theory

• Each lymphocyte has a unique receptor with specific characteristics.
• Foreign antigen interaction with the lymphocyte receptor leads to lymphocyte activation.
• Activated lymphocytes proliferate (increase in number) and differentiate into effector cells.
• These effector cells have identical antigen receptors to the original parent cell.
• Lymphocytes with self-molecule receptors are deleted early in development, ensuring they are absent from mature lymphocytes.

B Cell Types

• There are three major types of B cells.
• B1 cells are located in the peritoneal and pleural cavities.
• Marginal zone B cells are found in the marginal zone of the spleen.
• Follicular B cells reside in B cell follicles within lymphatic tissue.

Antibody Diversity

• Each B cell produces an antibody with unique specificity.
• The collection of antibody specificities is called the antibody repertoire.
• Humans possess approximately 10^11 antibody specificities.
• Encoding each specificity via separate genes is inefficient.
• Antibody diversity arises from somatic gene recombination and somatic hypermutation.

Somatic Gene Recombination

• DNA encoding antibody variable and constant regions is separated in all cells except B cells.
• During B cell development, coding segments are brought together to create functional antibody molecules.
• This occurs for both heavy and light chains.
• Light chain genes, like kappa (κ) and lambda (λ), exist.
• Kappa light chain genes are located on chromosome 2.
• Lambda light chain genes are on chromosome 22.
• The light chain V region is derived from combining V (variable) and J (joining) gene segments.
• The V gene segment encodes the first amino acids (95-101).
• The J gene segment codes for the rest of the domain (~13 residues).
• V region DNA is separated from the C region DNA by non-coding sequences removed through RNA splicing.
• For kappa light chains, approximately 38 V segments and 5 J segments exist.
• For lambda light chains, roughly 30 V segments and 4 J segments are observed.
• A variety of V regions potentially are produced (e.g., 190 for kappa and 120 for lambda).
• Heavy chain genes comprise V, D (diversity), and J gene segments.
• This results in about 5,520 likely V regions and ~1.7 x 10^6 different antibody specificities from combining H and L chains.

Additional Diversity Mechanisms

• Imprecise joining of segments: Nucleotide additions or deletions during segment joining lead to variability.
• Non-functional rearrangements are eliminated.
• Somatic hypermutation: Point mutations in V regions of rearranged H and L chain genes occur in secondary lymphoid organs.
• High-affinity antibodies are generated during an immune response.

This quiz explores the role of B cells in the adaptive immune response, including their functions in antibody production and clonal selection theory. Delve into the mechanisms that enable B cells to recognize diverse pathogens and contribute to immunity. Test your understanding of cellular interactions within the immune system.