Progressive sensitization of circulating basophils against <i>Ixodes ricinus</i> L. antigens during repeated infestations of rabbits

The sensitivity of rabbit basophils to antigens from <i>Ixodes ricinus</i> females has been studied by a degranulation test. Observations of basophil numbers and degranulation were made on the 6th day of each of four sequential infestations. Maximal degranulation of cells was observed after challenge of cells with antigen at a concentration of 10⁶ and 10⁷ pg/ml. At these concentrations, during a 1st infestation, 21.8 and 23.6% of cells degranulated. During a 2nd infestation, these percentages increased (34.8 and 33.8%) and reached 59.8 and 63.8% by the 4th infestation. A plasma factor which partially blocks basophil degranulation, is described. This was already present during the 1st infestation, since in its presence the percentage of degranulation was reduced by 2.8 and 4.0% respectively on challenge with 10⁶ and 10⁷ pg antigen/ml. Inhibition was maximal at the 4th infestation (difference: 16.5 and 20.5%). Basophil sensitization and inhibition of the degranulation are thus both progressive phenomena. After 10–15 infestations on four other rabbits, 75. 0 and 79. 8% degranulation was obtained. The inhibition of degranulation by plasma was also greater (difference: 25. 5 and 27. 4%). IgG specific anti-<i>I. ricinus</i> antibodies were identified by indirect immunofluorescence. In two animals, they were detected at the 6th day of the 1st infestation. Subsequently, they were generally present for all the animals.