GENERATION OF IMMUNE DIVERSITY: LYMPHOCYTE ANTIGEN RECEPTORS– Both heavy and light chain chains of immunoglobulins and T cell receptors contain

variable regions that are extremely diverse and constant regions that are relatively consistent.

– The restriction of VLCL and VHCH expression to a single member of the chromosome pair in any given B cell or T cell is termed allelic exclusion. The presence of both maternal and paternal allotypes (allelic forms) is observed within a particular individual.

– The variable regions of immunoglobulins and T cell receptors are formed by the rearrangement (at the DNA level) of multiple genes that are then transcribed into a single mRNA transcript that includes both the variable and constant regions. The variable and constant regions are then brought together by splicing of the mRNA to produce a transcript that can be directly translated into a single polypeptide.

– DNA chromosomal rearrangement is responsible for a significant portion of epitope-specific diversity for T and B cell receptors. Rearrangement occurs at both the DNA and RNA levels by the removal of nucleotides (deletions) followed by reannealing or by inversion of certain DNA sequences. Additional variation comes from junctional diversity. In Junctional diversity, Terminal deoxynucleotidyl transferase (TdT) can add or remove exposed ends of DNA before annealing, producing additional variation in nucleotide sequence.

– Recombination enzymes or recombinases mediate the genetic rearrangement and recombination that generates the variable regions of TCR and immunoglobulin chains.

– TCR and immunoglobulin gene rearrangements occur in the early stages of T cell and B cell precursor differentiation, prior to exposure of the cells to antigen.

– Gene clusters encoding α and δ chains of the TCR are arranged such that all of the δ chain genes (Dδ, Jδ, and Cδ) lie between the Dα: and Jα: + Cα: α chain gene.

– Gene clusters encoding immunoglobulin κ light, λ. light, and heavy chains are found on different chromosomes.

– The initial or primary antibody response to epitopes is dominated by production of the lgM isotype.

– Memory B cells, in response to subsequent restimulation by antigen and interaction with T cells, undergo further DNA rearrangement to juxtapose their rearranged VDJ genes next to different heavy chain C region genes, thereby altering the immunoglobulin isotype (e.g., lgG, lgA, or lgE) produced. This is known as the isotype switch.

– Somatic hypermutation is the process whereby memory B cells are stimulated by subsequent exposures to the same epitope. Small point mutations occur in the DNA encoding their VL or HL regions during the rapid proliferation that follows restimulation.

– Affinity maturation is the process whereby the binding of antibodies to a given antigen becomes better over multiple exposures. It is caused by the accumulations of small mutations that may affect the antigen-binding sites and the positive selection of those cells carrying mutations that result in tighter binding.

SIGNAL RESPONSE COUPLING: SIGNAL TRANSDUCTION MOLECULES – Leukocytes use receptors to sense their extracellular environment. Ligand binding by a

receptor leads to a signal transduction from the receptor-bound ligand to the nucleus involving phosphorylation of tyrosine residues. Two often-used tyrosine kinase signaling pathways use JAK-STAT and Ras-MAP kinase.

Page | 1/3

JAK-STAT pathway– Many extracellular stimuli activate a JAK (an acronym that stands for "Janus kinase" or

sometimes "just another kinase")-STAT (signal transducers and activators of transcription) signal transduction pathway.

– Ligand (e.g., cytokines, growth factors)-binding induces receptor polypeptides to dimerize and bind cytosolic. Activated JAKs are tyrosine kinases that phosphorylate tyrosine residues within the intracellular portion of the receptor chains.

– The phosphorylated tyrosine residues provide docking sites for SRC homology 2 (SH2) domains of inactive, cytosolic STAT molecules.

– Receptor-bound STAT molecules are tyrosine phosphorylated by the receptor-associated JAKs, allowing the STATs to disassociate from the cytoplasmic tail and dimerize with another tyrosine phosphorylated STAT.

– The STAT dimer translocates to the nucleus, where it binds to specific DNA response element(s) to regulate tyrosine phosphorylated gene transcription.

Page | 2/3

Ras-MAP kinase pathway – This pathway is named for Ras, guanosine triphosphate (GTP)-binding protein and MAP

or mitogen-activated protein. Following ligand-receptor binding, receptor dimerization promotes the phosphorylation of intracellular tyrosine kinase domains on the cytoplasmic tail of a catalytic receptor with intrinsic tyrosine kinase activity or allows activation of receptor-associated tyrosine kinases such as JAKs.

`

Page | 3/3