 Imagine you've been involved in an accident, you're bleeding and are rushed to hospital. Thankfully you're given a blood transfusion to replace your lost blood, but you can't just have blood from any old donor. In fact, when doctors first attempted blood transfusions in the 17th century, they were unsuccessful and patients died. How do medical teams now know what blood to give to which patient? Different people have different blood types or groups. You can be either O, B, A or AB. If you're given a blood transfusion, you must have blood which is compatible with your blood type. So if you're one of the AB blood groups, you can receive blood from all or lots of donors. Whereas if you're blood type O, you can only receive blood from other O's or if you're O-negative, you can only receive blood from other O-negative donors. But what makes blood into the different types? The type O, B, A or AB depends on little molecules called antigens that are found on the surface of the red blood cell. The antigens can be one of four options. A antigens, B antigens, both A and B antigens, or there may be no antigens. As you can see, the type of antigens you have determines which blood type you are. See how type O doesn't have any antigens. That's why it's the special blood type that can be donated to everyone else. Known as the universal donor, antigens trigger an immune response in which your body tries to get rid of them as it thinks they're foreign and bad. Because blood type O doesn't have any antigens on its surface, this means that when type O is donated to people from other blood groups, the recipient's body doesn't recognize them as foreign and so don't try to fight them off. Which is why no antigen O is the universal donor. Which type of antigen is presented on your red blood cells is determined through inheritance. You inherit one copy of the antigen G type or allele from your mother and one from your father to give you your genes, which then determines your blood type. See here how A O and B O end up as blood types A or B. That's because the allele from the antigens A and the allele from the antigens B are both dominant over the allele for no antigens O. Whereas C here, if you have one parent as A and the other as B, so you have genes AB, then you'd be blood group AB. This is an example of co-dominance, which we learn about more in detail in this video. To have blood group O, you need your genes to be OO and so both of your parents must also be O's. This is because the O allele is recessive. Despite being recessive, blood group O is the most common around the world. In summary, our blood group is determined by the presence or absence of A and B antigens on our red blood cells, which are inherited from our parents. The blood group you are also determines who you can receive blood donations from and who you can donate blood to. And thankfully, now that doctors understand this, patients can safely receive life-saving blood transfusions.