CD31 is a cluster of differentiation molecule. It is also called PECAM-1 for platelet endothelial cell adhesion molecule. It plays a key role in removing aged neutrophils from the body. Macrophages palpate any passing neutrophil and have to decide whether the cell is healthy or has to be ingested. In humans, the gene encoding CD-31 is found on chromosome 17.

If a neutrophil is dying, it will have phosphatidyl serine in its plasma membrane. Both the neutrophil and the macrophage express CD-31 on their membranes, and, during the testing process, these CD-31 molecules bind the two cells together. If the neutrophil is healthy, it will now fire a signal to the macrophage, and the CD-31 molecules will detach. The neutrophil is now free.