The CD11a (HI111) antibody recognizes the 180-kDa integrin α chain associated with CD18, the 95-kDa integrin β2, which form heterodimer glycoprotein called the lymphocyte function antigen-1 (LFA-1, CD11a/CD18). Leukocytes express four CD11/CD18 integrins which include a common β2 subunit and distinctive α chains. Besides CD11a/CD18, the other three integrins are known as CD11b/CD18, CD11c/CD18 and CD11d/CD18. LFA-1 is expressed on all leukocytes and is responsible for adhesion between leukocytes during immune response and transmigration of leukocytes across endothelium. The ligands for CD11a/CD18 are intercellular adhesion molecules CD54 (ICAM-1), CD102 (ICAM-2) and CD50 (ICAM-3). Patients bearing mutated CD18 genes do not express cell membrane LFA-1, a condition known as leukocyte adhesion deficiency 1 (LAD-1).