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

Rats were infected with Nippostrongylus brasiliensis larvae and immediately afterwards injected i.d. with rat serum containing anti-N. brasiliensis 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 Nippostrongylus. 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 in vitro, 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.