Who is not eligible to donate blood?

Here's a good start: It's not complete. For example, when I was a student pilot, I was deferred because of concerns about handling low oxygen. http://www.redcrossblood.org/donating-blood/eligibility-requirements

Go to blood bank: At the blood donation center they will screen your eligibility. In my experience, the most frequent reason to be vigilant is iron deficiency -- from frequent blood donation and/or in women coupled with heavy menses or recent childbirth. Oral iron supplements are an easy way to correct this but be sure you know why the iron was low- bleeding from ulcers or polyps or cancers needs exclusion.