Influence of the genetic background and parasite load of mice on the immune response developed against nymphs of Ixodes ricinus.

The immune response of BALB/c (H-2d), DBA (H-2d), C57BL/6 (H-2b), C3H (H-2k), CBA (H-2k), SJL (H-2s), and FVB (H-2q) mice infested once with 15 nymphs of Ixodes ricinus is polarized toward Th2 as suggested by cytokines produced by lymph node cells stimulated with concanavalin A. The parasite load does not influence the polarization of the immune response as observed in BALB/c mice, which developed a Th2 response when infested with 5 or 45 nymphs. As assessed by attachment and weights of engorged nymphs, no resistance was acquired by BALB/c, C57BL/6, or C3H mice undergoing three successive infestations. However, these mice produced a gradual increase in IgE.