How does the body respond to blood donation; are there any possible risks?
My answer below directly (and solely) addresses what I think is the main gist of the question: Are there any adverse effects on one's physiology as a direct result of donating blood? 

Blood donation, at the amounts typically taken (200~500mls), will not harm you. A healthy human being produces approximately 100,000,000,000 (one hundred billion) NEW hematopoietic cells DAILY to replenish short-lived blood cells and platelets. All of these ultimately derive from the hematopoietic stem cells residing in the bone marrow within one's long bones.

As you can appreciate, with such a huge amount of turnover daily, the body is constantly churning out new blood cells. Thus, any depletion in blood cells one will experience as a result of blood donation is a fraction of what will be lost normally in daily homeostasis anyway and thus will not affect a healthy individual.

And then there are the knock-on benefits suggested by epidemiological studies, some of which are alluded in this layman article: http://news.health.com/2014/06/13/4-unexpected-benefits-of-donating-blood/

[source: I'm a hematologist]