The clone 4D3, also called 158-4D3, a mouse monoclonal antibody, recognizes a protein of 205 kDa-220 kDa cell surface isoform protein known as CD45RA of the leucocyte common antigen (LCA) of the CD45 antigen family. The CD45RA antigen is expressed mostly on naive CD4+, CD8+, γδ+ T, and on most B and NK lymphocytes. Generally, the CD45RA antigen density decreases upon activation.