There are four types of blood in humans. Type A, B, AB, and O. The type is due to the presence of antigens(proteins) found in the blood. Type A contains antigen A. Type B contains antigen B. Type AB contains both antigens. Type O contains neither antigen A nor B. Blood type is determined by one gene that each parent donates to their offspring. In terms of blood transfusions, a person always can get a donation of their own blood type or type O. Type O is the universal donor because their blood lacks antigen A and antigen B. If someone receives blood with the wrong antigen for their blood type, an immune response occurs and clumping will occur. However, this immune response won't occur with type O blood, so it can be donated to anyone. Type AB is called the universal recipient. That is because this type contains both antigen A and B and any type will be familar to their system, so no immune response occurs. Not only are there four blood types, the blood can be Rh positive or Rh negative. Rh positive blood contains another blood protein, the Rh factor and Rh negative blood does not have this protein present. Most people are Rh positive, since this is a dominant trait. So, a person not only is categorized by blood type, but, also, whether that blood type is postivie or negative.
Scientists and doctors classify blood in different types or groups base on presence or absence of particular red blood cell antigens. There are several alternative system for classifying blood types. Among these the most widely used system called ABO system classifies human blood into four main groups based on presence or absence of two main antigens called A and B on the surface of the red blood cells. These blood types are:
- A type: This type has only A type antigens
- B type: This type has only B type antigens
- AB type: This type has both A and B type antigens
- B type: This type has neither A or B type antigens. This is the common blood type.
Another major system of blood classification is called Rh type classification. Blood with Rh antigens on the blood cells is called Rh positive, and that without Rh antigens is called Rh negative.
The blood of different people varies as it contains different molecules of protein called antigens and antibodies. The antigen protein molecules are found on the surface of red blood cells and antibodies are in the blood plasma. Antigens and antibodies vary between blood types. Due to variations in antibodies people have A, AB, B and O blood groups. Due to variations in the antigens blood is either Rh positive or Rh negative.
The variation in the types of blood is very important to consider when blood is being transfused between people. Giving the wrong blood type can lead to serious health problems.