The H5 monoclonal antibody specifically binds to CD307c which is also known as Fc receptor-like 3 (FCRL3), Immune receptor translocation-associated protein 3 (IRTA3), IFGP family protein 3 (IFGP3), and SH2 domain-containing phosphatase anchor protein 2 (SPAP2). The FCRL3 gene is present in humans but not in mice. CD307c is a type I transmembrane glycoprotein that belongs to the FCRL family within the Ig gene superfamily. CD307c shares sequence homology with classical the Fc receptors. CD307c is expressed on subsets of B cells, plasma cells, NK cells, CD8+ T cells, and CD4+ natural T regulatory cells. CD307c isoforms contain multiple extracellular Ig domains and immunoreceptor-tyrosine activation (ITAM) and inhibitory (ITIM) motifs in their intracellular domains. CD307c may be involved in the regulation of immune responses. Genetic variation in FCRL3 has been associated with susceptibility to certain autoimmune diseases.