The answer is genetic drift.
The correct answer is that "the T cell enters a state of anergy".
The activation of T cells requires two signals: (1) antigen specific signal presented by an antigen presenting cell (either a macrophage or a dendritic cell) that activates t cell receptors and (2) co-stimulatory signals that is not antigen specific but rather found in the plasma membrane of the antigen presenting cell (i.e. CD28). In the absence of a co-stimulatory signal, the t cell will enter a state of anergy or the inability to produce an immune response toward an offending antigen.
Radiation can actually affect the entire body, however it has a large effect of the bone marrow. Radiation is dangerous because too much can cause cellular depredation, this happens in environment that rapidly change like the bone marrow, which then interferes with blood cell production.
