The Duffy glycoprotein is a receptor for chemicals that are secreted by blood cells during inflammation. It also happens to be a receptor for Plasmodium vivax, a parasite that invades red blood cells (RBCs) and causes malaria. RBCs that lack the Duffy antigens are relatively resistant to invasion by P. vivax. This has influenced the variation in Duffy blood types seen in populations where malaria is common.