Integrin alpha X beta 2, also called CD11c/CD18, p150/95 or complement receptor type 4 (CR4), is one of four beta 2 integrins. The non-covalent heterodimer of 150 kDa alpha X/CD11c and 95 kDa beta 2/CD18 integrin subunits is commonly used as a marker for dendritic cells (DC) and classically activated macrophages (M1), but is also expressed on hairy cell leukemias, with lower amounts on other myeloid cells and activated B, NK and some cytotoxic T cells (1‑7). Inflammation enhances surface expression of alpha X beta 2 in states such as obesity (adipose DC), asthma (lung DC) and hypertriglyceridemia (circulating monocytes), increasing the adhesive capacity of the cells and contributing to pathology (8‑10). The alpha X vWFA or I‑domain, which contains the adhesion sites, forms the N‑terminal head region with the alpha X beta-propeller and the beta 2 vWFA domain (1, 11). Like other integrins, alpha X beta 2 has multiple activation states (3). In the presence of divalent cations and "inside-out" signaling, alpha X beta 2 is fully active and extended. In the inactive state, the heterodimer flexes in the center at the alpha X thigh and calf domains and beta 2 I‑EGF domains, impeding access to adhesion sites (1). The 1097 aa mouse alpha X/CD11c ECD shares 87% aa sequence identity with rat, and 71‑73% with human, canine and equine alpha X, while the 679 aa mouse beta 2/CD18 ECD shares 91% aa sequence identity with rat, and 80-82% with human, bovine, canine, and porcine beta 2 ECD. Active alpha X beta 2 shares some adhesion partners with alpha M beta 2/CD11b/CD18, including complement opsonin fragment iC3b, ICAMs, vWF and fibrinogen, and is expressed on many of the same cells (4-14). However, alpha M beta 2 activity is often constitutive, while alpha X beta 2 activity requires cell activation (4-7). alpha X beta 2 also binds osteopontin, Thy-1, plasminogen, heparin, and proteins with abnormally exposed acidic residues (14-18). The adhesion events are important for proliferation, degranulation, chemotactic migration, and phagocytosis of complement-opsonized particles (5, 6, 12, 14, 15). Mutations of beta 2, especially in the vWFA domain, cause leukocyte adhesion deficiency (LAD-1) and susceptibility to bacterial infections (19).