On the hand, while in vivo depletion of CD8 + T cells in mice abrogated protection against C. psittaci infection, depletion of CD4 + T cells did not affect protection17 indicating that CD8 + T cells may play a more crucial role than CD4 + T cells in protection against C. psittaci. Although both T-cell-mediated and humoral immune responses are known to be elicited following chlamydial infection, a number of studies have demonstrated that the presence of antibodies during natural or experimental infection does not correlate with protection. Here, CD4 is linked to infection.