Involved in T-cell adhesion processes. Plays a role in a late step of leukocyte extravasation helping leukocytes to overcome the endothelial basement membrane. Acts at the same site as, but independently of, PECAM1. Homodimer. Interacts with PILRB. Widely expressed with high levels in lung, spleen, thymus, liver and spinal chord. Expressed on leukocytes and endothelial cell contacts. Detected in a wide range of T-cells, including CD4-/CD8- and CD4+/CD8+ thymocytes. Expression is much lower in peripheral T-cells than in thymocytes. Belongs to the CD99 family.