The HI100 antibody reacts with the human CD45 isoform known as CD45RA, a protein tyrosine phosphatase of ≥ 220 kDa. CD45 is one of the most abundant hematopoietic markers, and is expressed on all leukocytes (the Leukocyte Common Antigen, LCA). Various isoforms are generated and expressed in cell-specific patterns. With their broad cell distribution, CD45 isoforms are critical for many leukocyte functions, regulating signal transduction and cell activation associated with the T cell receptor, B cell receptor, and IL-2 receptor. Other forms of CD45, with restricted cellular expression, include CD45R (B220), CD45RB, CD45RO and others. The HI100 antibody is widely used as a marker for human CD45RA expression on naive and activated T cells, B cells, and monocytes.