CD4 is a glycoprotein found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic cells. CD4 is a co-receptor that assists the T cell receptor (TCR) in communicating with an antigen-presenting cell. CD4 also interacts directly with MHC class II molecules on the surface of the antigen-presenting cell using its extracellular domain. 

The Feline CD4 monoclonal antibody, clone CAT30A, was produced in ascites fluid, clarified by filtration through a 0.2 micrometer filter.