-ˈfōr, -ˈfȯr noun
( -s )
Usage: often attributive
Etymology: c luster of d ifferentiation antigen + 4
: a large glycoprotein that is found on the surface especially of helper T cells, that is the receptor for HIV, and that usually 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