Demonstration of ricin within the mammalian para-aortic lymph node. I. Comparison of the localization, after intramuscular injection, with three immunocytochemical methods.

Following a supralethal injection of ricin into thigh muscle of the adult rat, the toxin was demonstrated post-mortem in the para-aortic lymph node, ipsilateral to the side of injection. The relative merits of two immunoenzyme methods, peroxidase anti-peroxidase (PAP) and avidin-biotin-peroxidase complex (ABC) and a silver-enhanced immunogold method (IGSS) were assessed in the detection of ricin in the lymph node tissue. The toxin was clearly seen to be located in association with histiocytes found both within and lining the sinuses of the nodes and also, in some cases, in the subcapsular sinus of the node; the toxin was not demonstrable within lymphoid follicles by light microscopy. However, using electron microscopy and the IGSS technique, cells carrying discrete particles of gold could be visualized within follicular areas. The IGSS and ABC-peroxidase methods were both found to give excellent results without background staining at the light microscopy level. However, when these techniques were used prior to embedding and viewing by electron microscopy, the IGSS technique proved to be far superior.