In a real world, potential donors are deferred and become ineligible to donate blood, for several different reasons that seem to vary quite frequently.
For example, Canadian Blood Services reject donations from people (including vegetarians) who spent more than three months in the U.K. and France between Jan. 1, 1980 to Dec. 31, 1996. The reason being is variant Creutzfeldt Jakob Disease, also known as mad cow disease, spread through the U.K. and France within the above time period. Unfortunately this restriction limits plenty of healthy Canadians from donating blood.
NetCAD, the Network Centre for Applied Development is a clinic at the University of British Columbia that collects blood for research from all those who are deferred from donating for the purpose of transfusions. They call this clinic a deferred donation clinic.
The purpose of blood research is to improve the quality of blood products blood centers provide to hospitals and to come up with ways to extend the shelf life of all blood products.