Because of the known functions of Th2 cytokines, we examined T cell, macrophage, and antibody responses to P. murina. We extend our previous findings to demonstrate that neonatal CD4+ T cells responding to P. murina produce IL-4 later during infection than in adult mice, which corresponds to the delayed infiltration of T cells into the lungs and so delayed clearance of infection. Here, CD4 is linked to infection.