The antigen is CD69. The epitope is a 53kD molecule. Under reducing conditions it showed two bands of 29 and 32 kD. Antigen distribution: Isolated cells: > 90 % of activated human peripheral blood lymphocytes. Tissue sections: In tonsil the antigen is strongly expressed on CD4+ T-cells in the light zone of the follicle centre, on extrafollicular T-cells, and weakly on mantle zone B-cells. In the spleen, the antigen is found on CD4+ T-cells in the middle of the marginal zone, while in thymus most of the medullary thymocytes are strongly positive. Specificity: Human: CD69 Other: not tested.