Effects of tick infestation on the plaque-forming cell response to a thymic dependent antigen.

Strain-2 guinea-pigs were given two five-day infestations with Dermacentor andersoni larvae. Each exposure consisted of 100 larvae, and the first and second infestations were separated by a seven-day tick-free period. Tick-exposed animals were given an intravenous injection with sheep red blood cells (SRBC) at selected times during and after infestation: (a) the last, fifth day, of a first exposure, (b) the second day of a second infestation, (c) the fifth day of a second infestation and (d) four days after termination of the second infestation. Ability of these animals to respond immunologically to the SRBC injection was assessed by the direct haemolytic plaque-forming cell assay, a very sensitive test used to determine the number of spleen cells producing IgM to SRBC target cells. Strain-2 animals given SRBC at the end of an initial infestation, or during a second tick exposure, produced significantly fewer direct haemolytic plaque-forming cells than did uninfested controls given a similar SRBC immunization regimen. Spleen cells of animals administered SRBC on the fourth day after termination of a second infestation displayed a haemolytic plaque-forming cell response which did not differ significantly from that of uninfested controls.