B Cells II

B cell specificity and diversity are generated through antigen-independent and antigen-dependent mechanisms.

Pairing of Different Heavy and Light Chains
- Different combinations of heavy and light chains contribute to the generation of antibody diversity.
Recombination of V, D, and J Segments
- Genes encoding the variable regions of immunoglobulins undergo somatic recombination, involving V (variable), D (diversity), and J (joining) segments.
- This process is crucial for creating unique receptors on B cells.
Variability at Joints of Recombined Gene Segments
- Additional variability is introduced at the junctions where gene segments are joined together during the recombination process.
- This increases the potential diversity of antibodies produced.
P- and N-Region Nucleotide Addition
- Nucleotide additions at the junctions known as P (palindromic) and N (non-templated) regions further enhance diversity.

Somatic Hypermutation
- After the initial activation of B cells by antigens, somatic hypermutation occurs. This process introduces point mutations into the V region of immunoglobulin genes, allowing B cells to produce higher affinity antibodies.
Class Switching
- This is a recombination event that changes the class of antibody produced without altering the specificity for the antigen.
Affinity Maturation
- This is the process through which B cells with higher affinity for the antigen are selectively expanded, leading to an overall increase in the quality of the immune response.

Progenitor B Cells
- Characterized by the expression of surface markers such as CD45R (B220) and undergo rearrangement of immunoglobulin genes.
Mature B Cells
- Approximately 5 x 10^6 mature B cells are produced in the bone marrow per day.
- These cells express different forms of immunoglobulin (IgG) on their surface.

Naïve B Cells
- These B cells circulate in the bloodstream and are capable of responding to specific antigens when encountered in peripheral lymphoid organs.
Activation Process
- Upon interaction with T helper cells (TH) and antigens, B cells can differentiate into either plasma cells or memory B cells.
- It is noted that approximately 10% of B cells may undergo cell death if not activated, while around 90% will get activated.
Differentiation into Plasma Cells
- Activated B cells can become IgM-secreting plasma cells, and undergo class switching to produce other isotypes such as IgG, IgA, and IgE depending on the signals received.

Purpose and Function
- Somatic hypermutation increases the affinity of B cell receptors (BCRs) for their specific antigens by introducing point mutations in the V region of the light and heavy chains of antibodies.
AID Functionality
- Activation-Induced Cytidine Deaminase (AID) plays a crucial role by introducing nicks in the DNA of the Ig genes during the process.
- This requires the presence of single-stranded DNA which is generally found in actively transcribed Ig genes.

B Cell Activation
- After exiting the bone marrow, B cells need to be activated through specific encounters with antigens.
- Without antigen stimulation, B cell lifespan is notably short.
T Cell Dependence
- There are two pathways for B cell activation:
  - T-Cell Dependent Pathway: Requires helper T cells for the activation process.
  - T-Cell Independent Pathway: Does not require T cell assistance for activation, often triggered by polysaccharide antigens.
**Signal Transduction
- Signal 1: Occurs through antigen binding to the B cell receptor (BCR), which must cross-link due to high concentrations of antigen.
- Signal 2: Derived from cell-cell interactions and cytokines; important in conditions of low antigen concentration.
Antigen Presentation
- Antigen-antibody complexes are internalized via receptor-mediated endocytosis and degraded to peptides that bind to MHC class II molecules for presentation.

Role of T Cell in B Cell Activation
- The interaction between CD40 on B cells and CD40L on T helper cells is pivotal in providing the second signal needed for B cell activation.
- The presence of B7 on B cells interacting with CD28 on T helper cells brings additional co-stimulation.
Cytokine Release
- Activated B cells begin to express receptors for various cytokines, facilitating further development and differentiation into plasma cells.

Location of B Cell Proliferation
- Proliferation predominantly occurs in the germinal centers of lymph nodes shortly after antigen exposure (7-10 days).
Germinal Center Functions
- The major outcomes of germinal center responses include affinity maturation, class switching, and the formation of plasma cells and memory B cells.
Pathways of Proliferation
- While affinity maturation and memory cell formation typically require germinal centers, some degree of class switching and production of plasma cells can occur outside of these specialized regions.

Different Classes of Antibodies
- Major classes include IgM, IgD, IgG, IgA, and IgE, each serving unique functions in the immune response.
Mechanisms of Action
- Antibodies can neutralize pathogens, opsonize microbes for phagocytosis, activate complement systems, and facilitate cellular responses.
Cytokines in Class Switching
- Various cytokines such as IL-2, IL-4, IL-5, IFN-γ, and TGF-β are involved in determining the antibody class and inducing class switching necessary for effective immune responses.

Processes Involved
- B cell activation leads to new DNA synthesis, cell division, transcription/release of cytokines, somatic hypermutation of Ig genes, and possibly mRNA processing related to antibody secretion or class switching.