The monoclonal antibody I21/7 reacts with the alpha subunit of mouse CD11a, also known as Leukocyte Function-associated Antigen-1 (LFA-1) and a member of the integrin family of cell adhesion molecules.1, 2 The antigen is expressed on thymocytes, T and B lymphocytes, and on non-lymphoid cells in the bone marrow.2 CD11a appears to be an essential component for T-cell activation by antigen-presenting cells and is important for cell-mediated killing. Co-stimulatory signaling by LFA-1 is achieved by pairing with its ligand, CD54/ICAM-1.1, 2