MultiDots consist of several quantum dots within a patented PEG polymer coating. The pegylated fluorescent nanoparticles have very tight size distribution and the amine functional groups on the surface allows conjugation of proteins to label cells of interest. MultiDots with a tighter emission spectrum than current fluorescent reagents, reduces spectral overlap, increases assay sensitivity and eliminates complex spectral compensation. The SK3 monoclonal antibody reacts with human CD4, a 59-kDa cell surface receptor expressed by a majority of thymocytes, a subpopulation of mature T helper cells, and, in low levels, monocytes. CD4 is a receptor for the human immunodeficiency virus (HIV). SK3 blocks HIV binding and mixed lymphocyte reaction. The SK3 and RPA-T4 monoclonal antibodies do not cross-block binding, suggesting recognition of distinct epitopes.