I raised the point of logic because, in a way, it obviates the need to bring "selfishness" into the discussion. That is, whether an act is selfish or not, relates only to the motivations of an individual, not the true, larger meaning of an act. So, as you say, both giving blood and not giving blood can be selfish. But, using the word "selfish" tells us nothing about the greater meaning of an act or deed for society, i.e., whether it is good or evil or in the spectrum there between. Note that the word "selfish," Ayn Rand notwithstanding, is usually pejorative. So, in the case where giving blood is motivated by selfishness, this does not negate the "goodness" of the act for society.