There are many benefits of blood donation. Most importantly, you save a life. Then of course, there are health benefits such as reducing the blood volume and pressure temporarily, thereby stimulating all the compensatory and defense mechanisms. You stimulate your body to produce new erythrocytes (red blood cells) and also lose calories in the process. You may unconsciously be protected from certain rare conditions like hemachromatosis (excessive iron in the blood) or polycythemia (increase in red blood cells).
A healthy person can safely donate blood up to four times in a year, leaving a gap of about three months between every blood donation episode. This time gap is essential for the body to regain its normal hemoglobin level and the numbers of various blood cellular components.