Short answer: no.
After natural disasters or terrorist attacks, emergency services rush to help injured. Injured are transported to hospitals which often run out of their stacks of blood. They ask people to donate their blood. That way they get high amount of blood in a short period of time.
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Does this good Samaritan deed harm your health? Short answer: No.
More than 9 million people donate their blood in the USA alone. Research going back to the 1980s has found a short-term drop in certain immune cells, but no long-term effect.
"Blood returns to normal in a few weeks." says Gustaf Edgren, a hematologist at Karolinska University Hospital in Sweden. "If the body needs more antibodies in the bloodstream, it will produce them extremely quickly."
Many donors believe that giving blood makes them healthier. Though that fell out of favor a century ago, the act of shedding blood to boost health has seen an uptick since the 1980s with the advent of the so-called iron-store hypothesis.This posits that too much iron in the body can increase a person's risk for cancer and coronary heart disease.
As evidence, adherents point out that women get cancer less often that men, and have lower rates of heart disease - at least until menopause- because they menstruate once a month, which lowers iron levels, and thus would offer natural protection.
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Still, there's no credible evidence that donating blood improves well-being either, though lots of donors persist in believing they gain benefits from it. and that's OK. "Perception of health is a large fraction of being healthy." Edgren says. "So that's a good thing.