context: While everyone's blood is made up of the same basic parts, there's actually a lot of variety in the kinds of blood that exist. There are eight different blood types, and the type you have is determined by genes you inherit from your parents. Most people have about 4-6 liters of blood. Your blood is made up of different kinds of cells that float in a fluid called plasma: â Your red blood cells deliver oxygen to the various tissues in your body and remove carbon dioxide. â Your white blood cells destroy invaders and fight infection. â Your platelets help your blood to clot. â Your plasma is a fluid made up of proteins and salts. What makes your blood different from someone else's is your unique combination of protein molecules, called antigens and antibodies. Antigens live on the surface of your red blood cells. Antibodies are in your plasma. The combination of antigens and antibodies in your blood is the basis of your blood type. There are four major blood groups and eight different blood types. Doctors call this the ABO Blood Group System. The groups are based on whether or not you have two specific antigens -- A and B: â Group A has the A antigen and B antibody. â Group B has the B antigen and the A antibody. â Group AB has A and B antigens but neither A nor B antibodies. â Group O doesn't have A or B antigens but has both A and B antibodies. There's also a third kind of antigen called the Rh factor. You either have this antigen (meaning your blood type is â Rh+â or âpositiveâ), or you don't (meaning your blood type is â Rh-â or ânegativeâ). So, from the four blood groups, there are eight blood types: â A positive or A negative â B positive or B negative â AB positive or AB negative â O positive or O negative Blood groups were discovered in 1901 by an Austrian scientist named Karl Landsteiner. Before that, doctors thought all blood was the same, so many people were dying from blood transfusions. Now experts know that if you mix blood from two people with different blood types, the blood can clump, which may be fatal. That's because the person receiving the transfusion has antibodies that will actually fight the cells of the donor blood, causing a toxic reaction. In order for a blood transfusion to be safe and effective, it's important for the donor and the recipient to have blood types that go together. People with blood group A can safely get group A blood, and people with blood group B can receive group B blood. It's best when a donor and recipient are an exact match and their blood goes through a process called crossmatching. But the donor doesn't always need to have the exact same type of blood as the person receiving it. Their types just have to be compatible. Type O negative red blood cells are considered the safest to give to anyone in a life-threatening emergency or when there's a limited supply of the exact matching blood type. That's because type O negative blood cells don't have antibodies to A, B or Rh antigens. People with O negative blood were once called âuniversalâ red cell donors because it was thought they could donate blood to anyone with any blood type. But now experts know there can even be risks with this type of blood. Over the past decade, there have been many claims about a so-called âblood type diet,â in which you eat specific foods for your blood type in order to lower your risk of certain diseases and improve your overall health. There is no scientific evidence that eating for your blood type makes you any healthier.