Detection of antigen presenting cells and T lymphocytes in the mid intestine. Microphotographs of mid intestinal sections from fish fed soybean oil stained with antisera against MHC class II (A) and CD3ϵ (B). Positive cells visualized with red colour were found in the epithelium and only scattered cells were present in the lamina propria. The density and distribution of positive cells were independent of diet. Scale bar 50 μm.