CD18, also known as the β2 integrin subunit, is a crucial component of various leukocyte integrins. CD18 is involved in a multitude of immune functions, including cell adhesion, migration, phagocytosis, and signaling. It is particularly important for the recruitment of immune cells to sites of inflammation and their interaction with the endothelium. The CD18 integrins also play a role in the recognition of microbial pathogens and contribute to the removal of pathogens following complement activation. Mutations or deficiencies in CD18 can lead to leukocyte adhesion deficiency, a condition characterized by recurrent infections due to the impaired ability of white blood cells to adhere to and migrate through endothelial cells. Additionally, CD18 plays a role in various immune-mediated diseases, and its expression can be modulated in different pathological conditions, such as cancer and autoimmune diseases. CD18 integrins are also involved in signaling pathways. For instance, CD11b/CD18 (Mac-1) can initiate signaling via its linkage to the actin cytoskeleton and associated signaling proteins.