CD11b also called integrin alpha-M (ITGAM) is one protein subunit that forms together with CD18 the heterodimeric integrin αMβ2 complex, also known as macrophage-1 antigen (Mac-1) or complement receptor 3 (CR3) (1). αMβ2 is expressed on polymorphonuclear neutrophils (PMN), monocytes, macrophages, some subsets of cytotoxic T lymphocytes, and NK cells. Antibodies against CD11b are frequently used to identify macrophages and microglia, however not all tissue-resident macrophages are CD11b positive. In the murine liver CD11b expression is rare and almost exclusively found on F4/80 negative cells (2). In the murine spleen CD11b+ cells are less numerous than F4/80+ cells, co-expression of CD11b and F4/80 is also rare and CD11b+ cells tend to be closer to the marginal zone (2). CD11b upregulation on residential alveolar macrophages is a marker of acute and chronic lung inflammation in mice (3). Also, in the brain CD11b is markedly increased during microglial activation (4).