Scientists here demonstrate the connection between reduced dietary protein and a better immune response, already known from the practice of calorie restriction: "Manipulation of dietary variables is one the most described events to retard the aging process and maintain immune function. The present study deals with the effect of variable dietary protein-carbohydrate ratios (without caloric restriction) on the alteration of immune response of male albino rats ... These results thus suggest that diets with variable dietary protein-carbohydrate ratios act as an exogenous modulator of immune response with age and [a low protein] diet may be beneficial to slow down/reduce the impairment of immune response in aged individuals." For comparison, you might also look at studies of methionine restriction without overall calorie restriction. Greater control of diet over the years adds up, and every extra year of health gained can make a big difference when the pace of medical development is rapid.