CD221, also known as insulin-like growth factor 1 receptor (IGF1R), is a 135 kD transmembrane receptor. It is widely expressed on lymphoid and myeloid cells as well as non-haematopoietic cells. CD221 is a tyrosine kinase receptor and the ligands for CD221 are insulin-like growth factor 1 and 2 (IGF-1 and IGF-2), the polypeptide hormones similar in molecular structure to insulin. IGF-1/2 binding activates downstream PI3K and MAPK pathways and plays an important role in cell growth. Recent studies reveal its function of recruiting immune cells during tissue repair.