Unlike innate immune responses, which can begin within hours of the onset of an infection, adaptive ...

First, antigen needs to be transported to a nearby secondary lymphoid tissue, processed, and presented by antigen-presenting cells to naive CD8 or CD4 T cells for T-cell priming. Second, the number of T cells specific for a given pathogen will be only around 1 in 10,000 to 1 in 100,000 (104 to 106) of the circulating T-cell repertoire; thus it can take some time before the relevant T cells circulating through the secondary lymphoid tissues reach the tissue containing the antigen that will activate them. Finally, it takes several days for an activated T cell to proliferate and differentiate into a large clone of fully functional effector T cells.