Therefore, CD8+ T cells appear to be involved in the immune response after infection of calves and lambs with C. parvum. Conversely, in studies using MHC class I deficient mice, which lack CD8+ T cells, it was shown that mice were not susceptible to C. parvum infection whereas mice which lacked CD4+ T cells due to a MHC class II deficiency were highly susceptible to infection. This evidence concerns the gene CD4 and infection.