CD11b is expressed on the surface of many leukocytes involved in the innate immune system, including monocytes, granulocytes, macrophages, and natural killer cells. It mediates inflammation by regulating leukocyte adhesion and migration and has been implicated in several immune processes. CD11b is also a receptor for fibrinogen, factor X and ICAM1. Expression is found on spleen cells, bone marrow cells and thymocytes and is commonly used as a microglial marker in nervous tissue.