If you have anemia, you cannot give blood. But anemia is often a temporary condition that can be corrected with diet. We test your blood for iron content before your donation. The likelihood of this happening is minimal. You should eat before donating and drink extra fluids, especially after your donation.
Athletes can donate as regularly as anyone else.
Be sure to give your body a day to replace most of the volume you donated. Do not give blood on the day you are scheduled to compete. If your blood clots when it is mixed with anti-A serum, then you have type A blood. If your blood clots when mixed with anti-B serum, then you have type B blood. If your blood does not clot in the presence of either serum, then you have type O blood.