A review of the literature shows that the thymus is the major reason for aging of the immune system. A graph of the mouse thymus mass is linear with age after puberty. However, T cell function is determined by thymic hormone factors and serum thymic factors. The net result is that T cell function is so affected that there is a deficit in T cell responsiveness. For humans a graph of 1-I (I is immunological efficiency) versus age is a straight line on a semi-log plot. The slope of the line is about the same as the incidence of class I cancer incidence vs. age.