HB5 recognizes CD21, a type I membrane glycoprotein. Expression of CD21 in humans is found on B cells, follicular dendritic cells, subsets of epithelial cells, and thymic T cells. The primary function attributed to CD21 has been to amplify the B cell receptor (BCR)-mediated signal transduction in response to antigen recognition. To enhance the BCR mediated activation, CD21 associates with CD19 and CD81 in a B cell–specific signal transduction complex, where interaction of BCR with CR2/CD19 amplifies the signals. In addition, CD21 serves as a receptor for split products of complement protein C3, the gp350/220 viral coat protein of the EBV and the immunoregulatory protein CD23.
An antibody, specific receptor protein, ligand, nucleic acid sequence (e.g., primer, probe) or similar reagent, which through specific binding or chemical reaction with an intended target, is used to identify or quantify a chemical substance in a biological specimen for the development of a laboratory developed test (LDT) or commercial in vitro diagnostic medical device (IVD). Commonly referred to as an analyte specific reagent (ASR), it is neither labelled with instructions for use nor diagnostic or performance claims.
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