The CW-1.2 monoclonal antibody specifically binds to CD199 which is also known as Chemokine (C-C motif) receptor 9 (CCR9) or C-C chemokine receptor type 9 (C-C CKR-9). CD199 is a ~42 kDa seven-transmembrane protein and member of the G protein-coupled receptor 1 family. CD199 binds to CCL25, which is likewise known as thymus-expressed chemokine (TECK) or small inducible chemokine 25 (Scya25). CCL25 is selectively and constitutively expressed in the thymic cortex and small intestinal epithelium. CD199 is highly expressed on CD4+CD8+ double-positive thymocytes, and mature naïve CD8+ T cells, but not naïve CD4+ T cells, in the peripheral lymphoid organs. CD199 is also expressed on small intestinal B cells, and on subsets of memory CD4+ T cells and CD8+ T cells, and dendritic cells. The CCR9/CCL25 pathway plays important roles in T cell development and gut-associated immune functions. It is especially involved in the recruitment of CD8αα+ intraepithelial lymphocytes (IELs) and the homing of other lymphocytes to the gut. Dysregulation of this pathway is associated with inflammatory responses, such as inflammatory bowel disease (IBD) and celiac disease.