As you probably know, red blood cells (RBCs) carry oxygen and carbon dioxide in our blood. Mature red blood cells are the only cells in our bodies that don't have nuclei. Each RBC starts out with a nucleus when it's made in our bone marrow, but it loses it when it is mature. Then it is basically just a bag of hemoglobin.

Illnesses of our RBCs are usually called "anemias," but there are several types of them. For example, sickle-cell anemia is a genetic disease that makes RBCs change into a crescent shape when they have given up their oxygen at the tissues. This can cause clogs in our smallest blood vessels, the capillaries.

Another kind of anemia is caused by lead poisoning. The lead may come from old paint. Laws prevent the sale and use of lead paint now, but it is still present in many old buildings.

If a person doesn't get enough iron in their diet, they can't make enough hemoglobin and may have iron-deficiency anemia.

There's a good site about blood and blood disorders that is written for young adults at:

If you're a fan of crime shows, you know that we can get DNA samples from blood. How is that possible if RBCs have no nuclei? (The site above will give you a hint.)