MHC Class II antigens are heterodimers consisting of one a- chain (31-34 kDa) and one beta-chain (26-29 kDa). The epitope has not been further characterized. Antigen distribution Isolated cells: The antigen is found on dendritic cells, B-cells and macrophages. The level of antigen detected by ER-TR1, ER-TR2 and ER-TR3 differs from strain to strain (see table below). Tissue Sections: The antigen is found on B-cells, interdigitating cells and macrophages in peripheral lymphoid organs but is absent from T-cells. It is also expressed as a fine reticular pattern on stromal thymic cells of the cortex and as a confluent pattern on stromal thymic cells of the medulla. Specificity: Mouse: cells expressing MHC class II antigens Other species: negative on human, other unknown