CD11b, also known as integrin alpha M or CR3A, is a key component of the integrin family, primarily expressed on the surface of immune cells such as macrophages, monocytes, neutrophils, and natural killer cells. It forms a heterodimer with CD18 （integrin β2） to function as a receptor for various molecules, including the iC3b fragment of complement protein, fibrinogen, factor X, and ICAM-1. This receptor complex plays a crucial role in mediating cell adhesion, migration, and the uptake of pathogens and complement-coated particles. Additionally, CD11b is widely used as a marker for identifying macrophages and other immune cells in research. Its involvement in immune responses and cell signaling makes it an important target in studies related to inflammation, immune regulation, and certain diseases.