Decreased susceptibility to infection and decreased midlife cancer explained the increased mean lifespan low doses. Decreased infection and cancer relate to improved immune competence, confirmed by evidence of increased humoral and cell-mediated immune competence following low doses. Increased cell repair enzymes and enhanced immune competence are keys to understanding many physiologic effects of low doses. Also important are repair of cell membranes, altered enzyme concentrations, and changed metabolic priorities. The interaction of immune system functions are absent in radiation experiments with cells in culture.
Thymic hormones contribute to lymphocyte maturation and differentiation.
The radiation syndrome in guinea pigs was greatly improved by injection of thymic hormone preparations. The spleen is vital to immunity. Exposure of mice activated splenic cell proliferation. DNA synthesis of spleen cells was also stimulated. Low doses increased the mitogenic responsiveness of mouse spleen cells as well as mitogen induced proliferation and function in both animal and human T cells.