In the article, "branded with infamy," the author describes selling her blood in order to meet her financial obligations. why did the blood collection centers eventually refuse to take her blood?

The blood collection centers eventually refuse to take her blood because the frequency with which she gave blood had left her anemic.

Anemia is a condition that develops when your blood lacks enough healthy red blood cells or hemoglobin.

Therefore her heart must pump more blood to compensate for the lack of oxygen in the blood. The consequences are huge, leading to an enlarged heart or heart failure.