Normal and bursa-deficient chickens (obtained by surgery or 19-nortestosterone treatment) were immunized at 6 weeks with killed Brucella abortis. Antibody responses were assayed by a sensitive microagglutination test.

Chickens with bursa weight > 1000 mg showed a rapid increase in mean log2 total antibody titer from 7 to a maximum of 21 at day 5 after immunization. Bursaless animals showed an increase from 3 to a maximum of 15 at day 7. The antibodies were chiefly mercaptoethanol-sensitive (IgM). By day 9 the total antibody titers in the bursaless and normal animals were almost the same—14 and 16. Bursaless chickens showed a very low ME-resistant (IgG) antibody titer (2) at day 9 when the normal chickens had IgG titers of 9.

The IgG titers at day 9 of all animals, normal and bursa-deficient, were strongly correlated with the IgM titers at the time (day 2) of the first clearly detectable response. The log2 antibody titer on day 2 in turn was linearly related to log bursa weight from 6000 mg down to about 200 mg. When the bursa weight was below about 100 mg, the day 2 response was not different from that of bursaless animals.

The observed subnormalities of bursa-deficient animals are consistent with a decrease in a largely bursa-dependent early immune response (and IgM antibody level) which determines the later, more severe impairment of IgG antibody synthesis.

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