It was also demonstrated that the activation of CD4+ T cells (MHC class II) occurs in the draining lymph nodes of H. contortus-infected sheep during a challenge infection with larvae, whereas during O. ostertagi infection in calves, antigen-reactive T cell precursor numbers increased dramatically on days 7 to 14 post-infection but decreased to the control level in infected adult animals [62]. The gene discussed is CD4; the disease is infection.