Figure 1.

Loss of T-cell proliferative capacity after long-term antigen exposure. (A) Activation of naïve T cells results in different levels of expansion based on many factors, including affinity. Generally, this is more than a 1 × 10⁴-fold increase in the number of CD8 T cells specific for any epitope. (B) If the antigen is not cleared, over time, the numbers of T cells specific for an epitope declines. In some cases, T cells responding to high-affinity epitopes are completely deleted after long-term exposure. (C) Vaccination after chronic exposure to antigen, even in the case of viral nonself epitopes, is ineffective. High-affinity cells may not be present to respond to the vaccine, and expansion ranges from a two- to fivefold increase in epitope-specific cells. The number of T cells after vaccination is never restored to the number of T cells induced by the initial activation.