The OX-22 antibody reacts with CD45RC on pre-B lymphocytes, B cells, CD8+ T suppressor/cytotoxic cells, and a subset of CD4+ T helper (Th) lymphocytes. It weakly reacts with thymocytes. CD45RC is a high-molecular-weight isoform of CD45 (Leukocyte Common Antigen); its level of expression distinguishes subpopulations of CD4+ T cells with Th1-like and Th2-like effector functions. Levels of expression of CD45RC have also been reported to distinguish resting from activated T cells at various stages of maturation. CD45 is a member of the Protein Tyrosine Phosphatase (PTP) family: Its intracellular (COOH-terminal) region contains two PTP catalytic domains, and the extracellular region is highly variable due to alternative splicing of exons 4, 5, and 6 (designated A, B, and C, respectively), plus differing levels of glycosylation. The CD45 isoforms detected in the rat are cell type-, maturation-, and activation state-specific. The CD45 isoforms play complex roles in T-cell and B-cell antigen receptor signal transduction.