CD4 and infection: Enzootic pneumonia is clearly a delayed hypersensitivity (delayed-type hypersensitivity: DTH) that occurs in the lung, and from the experimental point of view, it occurs in two distinct stages: in the first well-known sensitization, where T lymphocytes are activated, expanded, and differentiated into memory TCD4 (+) and Th1 CD4 (+), secreting IL-2, IL-12, and IFNγ recruit macrophages in the area of infection; in the second stage of provocation (elicitation), the process begins with the re-exposure of the antigen recognizing the memory Th1 CD4 (+); the result is the DTH response [39,40].