n, often attrib  :  a large glycoprotein that is found esp. on the surface of helper T cells, that is the receptor for HIV, and that usu. functions to facilitate recognition by helper T cell receptors of antigens complexed with molecules of a class that are found on the surface of antigen-presenting cells (as B cells and macrophages) and are the product of genes of the major histocompatibility complex .
 
   
 
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