Identification of CD4 on human helper/inducer T cells expressing the 60,000 M.W. surface antigen (HLA class II reactive). CD4 is present in low density on monocytes. The CD27 antigen recognizes disulfide-linked dimer nerve growth factor (NGF) super family of cells expressing the 55 kDa surface antigen. CD27 acts as a co-stimulatory on T lymphocytes with its ligand of CD70.