CD18 is a 90-95 kD type I transmembrane protein also known as integrin β2 subunit, LFA-1 β subunit, and β2 integrin. CD18 non-covalently associates with CD11a, CD11b or CD11c. CD18 is expressed on all leukocytes. CD18 and associated α chains function in adhesion and signaling in hematopoietic cells.