ACAAI Review for the Allergy and Immunology Boards - American ...

IMMUNE MECHANISMS / 25Costimulation Between the Antigen Presenting Cell and the TLymphocyteThe costimulatory molecules are on the antigen presenting cells. Their ligands are on theT lymphocytes. There is no T lymphocyte response without costimulation. The Tlymphocyte that recognizes the antigen, but is not costimulated, is said to be in a state ofanergy (nonresponsiveness).Costimulators may produce activation or induce negative regulation of the immuneresponse (Table 1-14).The TCR comes in two forms: αβ and γδ.Key Factαβ TCR recognize a single antigen only in the context of an HLA molecule. The receptor hasno signaling ability and requires accessory molecules for signal transduction..αβ TCRStructure—It is a heterodimer of an α and β chain, each with two Ig-like domains. One isthe variable domain, antigen contact, and HLA contact. The second Ig-like domain is theconstant domain.Key FactThe combination of the chains and their Ig-like domains are analogous to the immunoglobulinFab fragment. These molecules undergo recombination for their diversity. Both chains havecomplementarity-determining regions (CDRs), and like in an immunoglobulin, the αβ CDR3region of the TCR imparts the most significant sequence variability.Key FactThe particular variable β or Vβ region of the TCR is the binding site for superantigen.