Widely expressed membrane-bound protein which acts as a ligand to stimulate an activating receptor KLRK1/NKG2D, expressed on the surface of essentially all human natural killer (NK), gammadelta T and CD8 alphabeta T-cells (PubMed:11491531, PubMed:11777960). Up- regulated in stressed conditions, such as viral and bacterial infections or DNA damage response, serves as signal of cellular stress, and engagement of KLRK1/NKG2D by MICA triggers NK-cells resulting in a range of immune effector functions, such as cytotoxicity and cytokine production (PubMed:10426993). {ECO:0000269|PubMed:10426993, ECO:0000269|PubMed:11224526, ECO:0000269|PubMed:11491531, ECO:0000269|PubMed:11777960, ECO:0000269|PubMed:9497295}. This is the function of ENSG00000204520 (MICA, MHC class I polypeptide-related sequence A).