The circulating immunoglobulins involved in protective immunity to the intestinal stage of Nippostrongylus brasiliensis in the rat.

Antisera taken from rats after one, two or three larval infections with Nippostrongylus brasiliensis or after one infection of adult worms were separated by chromatography and the fractions were tested for passive protective activity. Irrespective of the number or type of infections given, protection against the parasite was most frequently obtained with fractions of antisera containing predominantly 7Sγ1-globulin. γM- and γA-globulins sometimes appeared to contribute to protective activity, but γM-globulin was not the major source of protective antibodies in antisera taken after a primary infection either of adult worms or of larvae. 7Sγ2-protective antibodies were found only in a pool of antiserum taken after three infections. Rats were passively protected with antiserum or fractions of antiserum, which were free of reagins and of other identifiable anaphylactic antibodies. It appears that rats can be passively immunized with antisera which do not have anaphylactic activity.