MHC Class I (HI21) antibody recognizes class I major histocompatibility complex (or human leukocyte antigen) monomorphic antigen which is expressed widely on human nucleated cells such as lymphocytes, monocytes/ macrophages, dendritic cells and other hematopoietic and non-hematopoietic nucleated cells, and platelets. The expression of HLA-I antigen varies with cell types and decreases with cell maturation, e.g. low expression on mature granulocytes and almost null on mature erythrocytes. HLA-I antigen is also expressed on most cells of human leukemic cell lines, but not or low in Daudi and K562 cell lines and nerve cells. In addition to membrane HLA-I antigen, a soluble form of HLA-I can be detected in serum.