You wouldn't know it by looking at any one of us, but coursing through our veins every second of every day are tiny variations that categorize our blood into groups: A+, A-, B+, B-, O-, O+, AB+ and AB-. 

These minute differences usually don't matter until they matter -- when you're in the hospital in need of a blood transfusion, or after you've donated blood, and you learn which type you have. Some people find out during pregnancy, when special treatment is required for someone with a negative blood type.

But ongoing research into blood type suggests it may matter more than we think it does -- at least when assessing risk for certain health conditions, especially heart disease. These invisible differences in the blood may give some people an edge at staving off cardiovascular problems, and may leave others more susceptible.

What does blood type mean, and how are they different?