The HISTO TRAY test trays are based on the microlymphocyte toxicity test (MLCT). In this test, anti-HLA sera react with corresponding, membrane bound antigens of human lymphocytes. The addition of rabbit complement leads to structural changes of the cell membrane, so that an indicator dye can penetrate the lymphocytes and tint them (positive reaction). If an antigen-antibody reaction does not occur, the cell membrane remains intact. The cells are unable to absorb the dye (negative reaction).