Acquired resistance to ticks. II. Effects of Cyclophosphamide on resistance.

Guinea-pig developed resistance to Dermacentor andersoni larvae after one infestation. Cyclophosphamide administered in one dose (300 mg/kg) 48 hr prior to an initial infestation with larvae blocked the acquisition of resistance. When cyclophosphamide was given in a similar regimen to guinea-pigs which had already acquired resistance, the expressin of resistance was partially blocked. It was proposed that the blockage of the acquisition of resistance further confirmed the immunological nature of tick resistance. Partial blockage of the expression of resistance by cyclophosphamide substantiated the presence of a humoral component to the resistance mechanism. The presence of a cell-mediated component was previously well established.