Humoral immunity in the ewe. 2. The effect of pregnancy on the primary and secondary antibody response to protein antigen.

This study examines the effect of pregnancy on the quantity and isotype of antibody in ewes immunised with a novel primary antigen. The primary and secondary antibody levels to bovine serum albumin (BSA) in alum adjuvant, were compared between non-pregnant ewes and ewes immunised at different stages of pregnancy. Anti-BSA isotype specific responses were measured using an indirect ELISA. Results show the levels of immunoglobulin M (IgM) increased and persisted in response to BSA during pregnancy (P less than 0.05). Secondary immunoglobulin G1 (IgG1) titres were significantly impaired in late pregnancy and during lactation (P less than 0.05). The lower levels of immunoglobulin G2 (IgG2) were unaffected by pregnancy under these experimental conditions. Ewes were also immunised with BSA in alum adjuvant for their primary inoculum and BSA in different adjuvants for the secondary inoculation. Primary IgM persisted at higher levels during pregnancy compared with the response in non-pregnant ewes (P less than 0.05). Following the secondary injection, lower levels of anti-BSA specific IgM, IgG1 and IgG2 antibodies were produced in late pregnancy and during lactation compared with the levels in non-pregnant control ewes (P less than 0.05). Alterations in regulatory T-cell function or effector B-cell activity would most readily explain the qualitative changes in antibody titre observed following primary injection during pregnancy. The results also suggest an associated impairment of immunological memory following primary immunisation during pregnancy.