The correct answer is A. Antibodies.
Antibodies are proteins that are produced by the body’s immune system in response to foreign substances called antigens. Antigens can be found on the surface of bacteria, viruses, and other cells. When antibodies bind to antigens, they help to destroy the antigens or mark them for destruction by other immune cells.
Blood is grouped based on the presence or absence of certain antigens on the surface of red blood cells. The four main blood groups are A, B, AB, and O. People with type A blood have A antigens on their red blood cells, people with type B blood have B antigens, people with type AB blood have both A and B antigens, and people with type O blood have neither A nor B antigens.
In addition to the A and B antigens, there is another antigen called the Rh factor. People who have the Rh factor are Rh-positive, and people who do not have the Rh factor are Rh-negative.
Blood transfusions are only possible if the donor’s blood type is compatible with the recipient’s blood type. For example, a person with type A blood can only receive blood from a donor with type A or type O blood. A person with type B blood can only receive blood from a donor with type B or type O blood. A person with type AB blood can receive blood from any donor, and a person with type O blood can only receive blood from a donor with type O blood.
It is important to know your blood type in case you need a blood transfusion. You can find out your blood type by having a blood test.