Distribution of sheep erythrocytes as antigens in rat spleen.

Although sheep erythrocytes (SRBCs) are extensively used as an antigen in immunological studies, their histological distribution in lymphoid tissues has received little attention. The objective of this study was to determine the histological distribution of injected SRBCs in rat spleen. SRBCs were labelled with fluorescein isothiocyanate (FITC) to facilitate their identification in spleen sections with fluorescence microscopy. Rats received intravenous injections of FITC-labelled SRBCs and were sacrificed at various periods after injection. At 15 min, SRBCs were distributed throughout the marginal zone and red pulp. After 4 h, intact SRBCs were located mainly in the red pulp, while the marginal zone contained fluorescent flocculent material. At later periods this material was present in the periarterial lymphatic sheath (PALS) and in the light and dark zones of the germinal centers. By 12 h, the most intensely labelled areas in the white pulp were the crescent-shaped light zones. In 12 and 24 h, the PALS contained numerous foci of labelled granules. Some of the dark zones also contained label. After 48 h, the only areas containing label were the light zones of the germinal centers.