The Guava® Express CD3/CD4 Kit is a two-color direct immunofluorescence reagent kit for enumeration of mature CD4(+) T lymphocytes in human blood. The kit consists of a monoclonal anti-human CD3 antibody conjugated to the tandem dye phycoerythrin (PE)-Cy5 (PECy5), a monoclonal anti-human CD4 antibody conjugated to PE, Guava 1X Lysing Solution to lyse erythrocytes and Guava Fixative to preserve the cells. The CD3 antibody uniquely identifies T cells and recognizes an epitope that is expressed on the epsilon chain of the CD3/T cell antigen receptor (TcR) complex. The CD4 antibody allows the identification of human helper/inducer CD4(+) T cell (HLA Class II reactive) and recognizes a 60,000 Da MW surface antigen. However, the CD4 antigen is also present on monocytes but at much lower density. Monocytes also lack expression of the CD3 molecule.