Peripheral circulating allergens as a cause of loss of PCA reactivity in parasitized rats.

Rats were infected with larvae and immediately afterwards injected i.d. with rat serum containing anti- homocytotropic antibody. Subsequent i.v. challenge with allergen using the homologous passive cutaneous anaphylaxis (PCA) technique indicated that progressive desensitization and refractoriness to challenge occurred from day 8 onwards in the infected animal. Uninfected control animals were still PCA-reactive at day 20. The reaction appeared to be immunologically specific for . These findings were interpreted as indicating the presence of peripheral circulating allergen which pre-empted subsequent PCA reactivity. It is suggested that such circulating allergen could help explain refractoriness in some sensitized mast cells when examined , and lack of skin reactivity to nematodes in some infected animals and human beings, as well as the non-appearance of circulating homocytotropic antibody until after infection has been eliminated.