CD18, also known as integrin β2, is a 95 kDa glycoprotein that is a crucial component of several β2 integrins. These integrins are heterodimeric type I transmembrane proteins composed of a variable alpha chain （CD11a-d） and the common beta chain CD18. The CD18 protein is encoded by the ITGB2 gene in humans. It plays a significant role in cellular adhesion and immune responses. When CD18 binds with different alpha chains, it forms various heterodimers such as lymphocyte function-associated antigen-1 （LFA-1） with CD11a, and Macrophage-1 antigen receptor （Mac-1） with CD11b. These heterodimers are involved in processes like leukocyte adhesion to endothelial cells, phagocytosis, and immune cell activation. Deficiencies in CD18 expression can lead to adhesion defects in white blood cells, impairing the immune system's ability to fight infections.