CD11c, also known as the integrin alpha X chain, is a type I transmembrane glycoprotein and a component of the complement receptor 4 (CR4). CD11c is primarily expressed on myeloid dendritic cells (DCs) and acts as a positive regulator in the adhesion, migration, antigen recognition, and presentation processes, leading to the activation of CD4+ and CD8+ T cells. It is not only expressed on dendritic cells but also on other immune cells such as alveolar macrophages and cells in adipose tissue. In humans, monocytes and dendritic cells can both express CD11c. Additionally, CD11c expression has been detected in a subset of gut macrophages, neutrophils during inflammation, a small fraction of B cells (mainly in autoimmune diseases), natural killer cells, and T cell subsets. In diseased brains, activated microglia can also express CD11c. CD11c forms a heterodimer with CD18, belonging to the β2 integrin family, and together with CR3 (comprising CD11b and CD18), it is involved in cell adhesion, migration, and phagocytosis. CR3 and CR4 have overlapping ligand-binding specificities and bind to various ligands such as fibrinogen or heparin. In oncology, CD11c is an important marker for diagnosing hairy cell leukemia, acute myeloid leukemia, and some chronic lymphocytic leukemias. Furthermore, the expression of CD11c is associated with liver inflammation and microglial activation.