Local immunity and Ostertagia circumcincta: changes in the gastric lymph of immune sheep after a challenge infection.

Immunity to Ostertagia circumcincta was demonstrated in 5 previously infected sheep which were killed 10 days after challenge with 50 000 larvae. This immunity was expressed as a reduction in the number as well as the degree of development of the surviving parasites compared with those found in 6 control sheep. Gastric lymph was sampled from the immune group from 2 days before till 10 days after challenge and a secondary local immune response was detected. The main features of this response were a large increase in cell output in the lymph, especially in lymphoblasts and IgA-containing cells, which reached a peak on day 4 or 5, followed by a ten-fold increase in IgA immunoglobulin and IgA anti-worm antibody which reached a peak 7 or 8 days after challenge. The timing of these events suggested that the cellular, but not the IgA, response could have been involved in a putative mechanism which caused arrested development, although both components could have been implicated in mechanisms which may have caused expulsion of developing larvae.