CD4+ T helper 1 (Th1) cells are critical in controlling infection by production of the cytokine IFN-γ, which upregulates the expression of MHC-class II molecules on antigen-presenting cells (APC) thus enhancing their capacity to present antigen to activate CD4+ T cells, and also activates macrophages to kill intracellular pathogens1. Here, CD4 is linked to infection.