A method for the specific histochemical demonstration of antibody in cells and parts of cells is described. It consists of carrying out a two stage immunological reaction on frozen sections of tissues: (a) allowing reaction between antibody in the tissue and dilute antigen applied in vitro, and (b) the detection of those areas where this antigen has been specifically absorbed by means of a precipitin reaction carried out with fluorescein-labelled antibody. Examination under the fluorescence microscope reveals the yellow-green fluorescence of fluorescein over those areas where a precipitate has formed.

A study of the hyperimmune rabbit on the first few days after the last of a series of intravenous antigen injections reveals that antibody against human γ-globulin or ovalbumin is present in groups of plasma cells in the red pulp of the spleen, the medullary areas of lymph nodes, the submucosa of the ileum, and the portal connective tissue of the liver. Because of extensive non-specific reactions, the bone marrow could not be examined. Small amounts of antibody were occasionally visible in cells in the lymphoid follicles of the spleen and lymph nodes, so that a minor contribution by lymphocytes to antibody synthesis cannot be excluded.

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