Everybody has a blood type. The blood type you have depends on if you have these things called “antigens”. Antigens are little proteins found on the membrane of red blood cells. Think of them as little attachments on the cells in your blood. The antigens you have are hereditary, meaning they depend on the antigens your mom and dad had. There are A antigens and B antigens:
-If you have A antigens, you have type A blood.
-If you have B antigens, you have type B blood.
-If you have both A and B antigens, you have type AB blood.
-If you don’t have either antigen, you are type O blood.

Blood typing is very important because if you donate your blood to help others, it has to be the same type. If you give someone blood that is a different type, their body can reject it and it can be very dangerous. To determine blood type, doctors use antibodies. Antibodies are another little protein that can recognize specific antigens and attack them. They mix your blood with different antibodies that attack types A and B to see how it reacts. For example, if your blood cells clump together when mixed with the antibody that attacks type A, you are type B. If your blood clumps when mixed with the antibody that attacks type B, then you are type A. I’ll attach a picture of what it looks like below (that person has type B blood). blood type image.

Our blood types can be detected by looking at red blood cells. Red blood cells are the most common type of cells in our blood, and they are responsible for delivering oxygen to all parts of our bodies. These cells have different things on their surfaces, and these things are called antigens. Antigens are small biological things (smaller than cells) that, if recognized by a body as a threat, will cause antibodies to be made (antibodies that stick to the antigens and serve as tags for the immune system to destroy the antigen-producing cell). There are two different major types of antigens called A and B. Because of these two groups of antigens, there are four different types of red blood cells, those with antigen A only, those with antigen B only, those with both antigens A and B, and those with neither antigen A or B. After getting a small amount of blood from a person, doctors can use antibodies (made in labs) to detect whether this person has type A, type B, type AB, or type O (neither antigen) blood.

There is one other important type of antigen that the cell surface has, and that is the Rh antigen group. The most important one out of this group is the D antigen, and we either have the Rh D antigen or we don't. People with this antigen are classified as Rh+, and people without this antigen are classified as Rh-. With the ABO system discussed above, if we see someone's blood type marked as A-, then we know that this person has antigen A, but does not have antigen D from the Rh group of antigens.

Why are these antigens important? Sometimes, because of injury or illness, people need to get blood from others. While the antigens on the surface of red blood cells are not threats to the human body that makes these red blood cells, the antigens can be seen as threats by other human bodies. When the body sees threats, it will activate immune responses to get rid of these threats. Therefore, knowing and matching the blood types of both the blood donor and the blood receiver can prevent the receiver from starting an immune response and getting sicker.

Blood type is determined by the combination of antigens and antibodies (specific types of proteins related to immune response) in your blood. Blood types in humans are divided into 4 major groups, with two "types" of each. The major group is determined by the presence or absence of the antigens known as A and B. Someone with A antigens (and B antibodies) has the A blood type, someone with B antigens (and A antibodies) has the B blood type, AB has both A and B antigens but neither A nor B antibodies, and O type does not have the antigens but has both antibodies. The final component of blood type is determined by a third antigen called Rh. Presence this antigen is indicated by the additional designation of "positive" (e.g. type A positive blood), and absence by "negative".

One way to determine blood type is to see which antibodies attack it. Antigens allow the immune system to identify foreign cells which should be attacked and destroyed. Notice above that antigens and antibodies are opposite (e.g. A antigen with B antibody). This means that the immune system of a person with type A blood will see type B blood as foreign and try to destroy it. In the body, the foreign blood is destroyed through agglutination (clumping) of the foreign blood cells when the antibodies bind to antigens on two blood cells.

The same mechanism can be exploited in blood typing: a droplet of blood is placed onto a slide containing A antibodies and another on a slide with B antibodies. If the blood clots on the slide with A antibodies, the blood is type A; vice versa for type B; if it clots on both, then the blood is type AB, and on neither means type O. There is an additional antibody for the Rh antigen which works the same way.

[For some history about blood types, go here. ]

You need to get your blood tested for proteins on the surface of your blood cells.

You get tested to determine your blood type - ABO and Rh Factor - positive or negative. I'm A+ :-].