The phenomenon of eosinophilia was studied in rats using inoculation with Trichinella larvae as the experimental stimulus. Comparisons were made between the eosinophil response accompanying active infestation via the gastrointestinal tract and that resulting from parenteral inoculation of larvae or their products.
A vigorous eosinophilia could be provoked by a single intravenous injection of intact parasites. In this circumstance the larvae lodged in the lungs causing an acute inflammatory reaction which led to their disintegration within 24 hr. Intraaortic injection also produced a significant response, whereas inoculation of the same number of parasites by the intramuscular, intraperitoneal, or subcutaneous routes did not cause eosinophilia. Eosinophilia likewise failed to develop if parasites were homogenized before intravenous injection, so that they were not arrested in the lungs.
Antibody levels, as measured by a hemagglutination technique, using whole larval extract as antigen, did not correlate closely with the eosinophil response.
The findings are interpreted as suggesting that increased eosinophil production is induced under some circumstances as a consequence of interaction between intact parasites and certain host cells in blood and tissue. No evidence was found for the existence of a specific constituent of the parasite capable of stimulating eosinophil production.
Attention is directed to features of eosinophilia which fit with the concept that this phenomenon belongs in the category of immunologic reactions.