MICA (MHC class I polypeptide-related sequence A) and MICB (MHC class I polypeptide-related sequence B) are stress-inducible proteins that belong to the MHC class I family but do not bind to 尾2-microglobulin or present peptides. They are encoded by genes located within the HLA class I region of chromosome 6. MICA and MICB are expressed on the cell membrane in response to cellular stress or cancer transformation. These proteins act as ligands for the NKG2D receptor, which is expressed on most natural killer (NK) cells, CD8 伪尾 T cells, and 纬未 T cells. The binding of MICA/B to NKG2D can activate these immune cells, leading to cytotoxic responses against stressed or transformed cells. MICA and MICB are highly polymorphic, with over 106 alleles reported for MICA and 25 for MICB. This polymorphism may affect their affinity for NKG2D and, consequently, the activation thresholds of NK and T cells.