This antibody recognizes an extracellular epitope of CD4 antigen, a 55 kDa transmebrane glycoprotein expressed on a subset of T lymphocytes ("helper" T-cells) and also on monocytes, tissue macrophages, and granulocytes. This antibody does not block Leu3a and OKT4 binding. This antibody blocks HIV-1 infection in cell to cell system.