COATING of red blood cells with antigenic substances is used for titration of antibodies because the blood cells are a stable and uniform vehicle for the antigens1–3. In our experiments red blood cells of sheep were first treated with tannic acid (1:20,000 in saline, pH 7.2) to stabilize them. If the solution is stronger, spontaneous agglutination takes place. The protein (bovine γ-globulin) was coated on the cell at a pH 6.4. The cells were lysed in distilled water. The cell walls were then shadowed with gold-palladium, and studied in a Siemens Elmiskop I. Fig. 1 shows an untreated cell wall. The surface is fairly smooth. After treatment with tannic acid the surface becomes rougher (Fig. 2). Coating with protein produces a mottled appearance, and the outlines of the shadow of the edge of the cell wall are somewhat diffuse (Fig. 3). It is difficult to explain these morphological changes in terms of physical and chemical processes, especially as the arrangement of the molecules on the cell surface is not fully understood. Tannic acid acts presumably as a fixative like formalin and the micrographs show that a change has taken place on a sub-microscopic scale. It is possible that the protein adheres more readily to the rough surface. The roughening of the surface may also explain why a stronger solution of tannic acid produces agglutination of the cells. Judging by the electron micrographs it appears that the protein covers the cell wall in shapeless masses, which produce the mottled appearance.
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DEUTSCH, K. Coating of Red Blood Cells with Antigenic Substances. Nature 184, 278–279 (1959). https://doi.org/10.1038/184278a0
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