Age immunity to Ostertagia circumcincta: comparison of the local immune responses of 4 1/2- and 10-month-old lambs.

In 2 experiments, the local immune response to infection with 50,000 O. circumcincta was monitored in 4 1/2-month-old previously infected lambs by following changes in the composition of gastric lymph. The previously infected lambs were partially resistant to the challenge in that they harboured a greater proportion of early fourth stage parasites and that the developing larvae recovered from them were distinctly shorter than those present in controls. They also exhibited a secondary local immune response in the gastric lymph consisting of increases in the output of lymphoblasts and IgA cells which peaked 3 to 5 days after challenge, followed by increases in lymph IgA and IgA anti-larval antibody on days 6 and 7. The size of the total IgA response in a sheep was highly correlated with the degree of stunting of its larvae, suggesting that IgA antibody can reduce the growth of developing Ostertagia. When compared with the results of 2 identically designed experiments carried out previously in 10-month-old sheep, the younger lambs were measurably less resistant to challenge and most aspects of the magnitude of the local immune response were significantly depressed, although the timing of the response was similar. It is suggested that this poorly developed ability to mount a secondary mucosal immune response may explain the relative failure of the younger lamb to develop resistance to gastro-intestinal nematodes.