The YN1/1.7.4 antibody reacts with CD54, a 90 – 110 kDa type I transmembrane glycoprotein that is a member of the immunoglobulin (Ig) superfamily. CD54 consists of five Ig-like C2 type domains and has two isoforms generated from alternative splicing. As an important intercellular adhesion molecule, CD54 is involved in inflammatory reactions, antigen-specific immune responses, lymphocyte trafficking, and T cell signal transduction events. CD54 mediates important cell-cell interactions and adhesion reactions by binding to its major ligands, LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18). CD54 may also act as a receptor for rhinoviruses and some coxsackieviruses. CD54 is expressed on lymphocytes, monocytes, macrophages, dendritic cells, endothelial cells, and high endothelial venules. Expression of CD54 is upregulated on activated lymphocytes and also, following stimulation with inflammatory mediators such as cytokines, LPS, and IFN-γ.