At age 3 (and now at age 5) my DD loved "Your Body Battles" books by Vicki Cobb.
These are very conceptual, but very visual and well-written which makes them accessible for a gifted preschooler. They teach about antibodies, different types of white blood cells, etc and how they help the body heal. I learned a lot from these beautiful books too!

http://www.amazon.com/Your-Body-Battles-Cold-Vicki/dp/1580138365/ref=pd_bxgy_b_img_c