Blood transfusion is always a life saving process.When someone is in need of blood, we usually don't hesitate to give, especially when the person is our family or relative.There are people who donate blood and it is considered to be a very noble deed.But we know that our blood cannot be given to any person we like.The person's blood has to be matched with our blood and only if it is compatible we can donate our blood.So what is matched in this case?.Why an A group person cannot give blood to a B group person?.Why O group can be given to all persons in most cases?.These questions are answered by this article in a very detailed and simplified manner.

What are ABO blood groups?

To understand blood group matching we must first understand what is meant by ABO blood groups.We know that there are four groups namely, A group, B group, O group, and AB group.Here at this point, i am not considering the Rh factor which gives positive and negative to blood groups.To study about the blood group, we must first know the meaning of some basic terms in medical science.They are antigen and antibody.Generally antigen is referred to any foreign substance entering our body.To counteract this foreign material our body produce another substance called antibody.This is the normal immune response of the body and the best example is that of a virus entry and the antibody production to destroy that virus or antigen.So now you have a basic idea about antigen and antibody.But in the case of blood group, the meaning of antigen is little different.Here we refer to proteins inside our body itself as antigens and these proteins are seen in Red Blood cells or RBC.Obviously these are not foreign but our own molecules, but still we call it antigens.Now we have to consider this in detail.

What are blood group antigens?

We know that blood contain liquid plasma and three basic type of cells, namely Red blood cells or RBC, White blood cells or WBC, and platelets.Here we have to consider RBC and plasma.In RBC different types of proteins are present.In general these proteins can be called as blood group antigens.They are mainly found on cell membrane.It is said that there are more than 160 proteins present.But we consider mainly two antigens, which are called Antigen A and Antigen B.Specifically they are glycoprotein and they are much more important than any other proteins in RBC.Based on the presence or absence of these two antigens we name the blood groups.In other words when we say A group, only antigen A is present in the cell membrane of RBC.In B group only antigen B is present.So what will be the condition of AB group.Yes.your guess is right.It has got both antigens in RBC.Then what is the case of O group people.For them both antigens are absent in RBC.It is just like a zero group.So this is how blood group names are derived in a person.Now we have to consider about blood group antibody.

What are blood group antibodies?

Here we must consider blood plasma.In the blood plasma, blood group antibodies will be present which are named as antibody a and antibody b.Small letters are used to designate antibodies.The role of antibody in this case is similar to the antibody of our immune system.It means that antibody a will act against antigen A and antibody b will act against antigen B.Thus based on this fact there is a rule.A person with antigen A will not have antibody a, but will have antibody b and a person with antigen B will not have antibody b but will have antibody a.In case of AB group, since both antigens are present, no antibody will be present in the plasma and in case of O group, since both antigens are absent, both antibodies will be present.Now what is reason for this rule?.This condition ensures that our own antigen in RBC is not attacked by our own antibody in plasma.Now lets summarise:

1.Blood group A has antigen A and antibody b

2.Blood group B has antigen B and antibody a

3.Blood group AB has both the antigens and no antibody

4.Blood group O has no antigens and both antibody.

5.The name of the blood group is derived by the presence and absence of antigens.A and B.

Matching and consequences of mismatching

Let us take an example.A person giving blood is called donor and the person receiving blood is called recipient.Suppose the donor is B group and recipient is A group.When the B group blood reaches A group blood, the antigen B present in the donors blood will be considered as foreign body by the recipient blood and the antibody b in the recipients blood will attack antigen B.Vice versa also same thing happen.When the A group blood of donor mix with B group blood of recipient, antibody a of recipient will attack the antigen A.Now lets take the example of O group.Suppose the donors blood is O group.We know there are no antigens in O group.Thus no immune response will be made by any of the antibody in recipients blood.So now you know why O group is called Universal Donor.In case of AB blood you cannot give it to other groups, because it has got both antigen, and the antibody in recipients blood will attack at least one of the antigen.But since AB group don't have any antibody, if the recipient is AB, the person can receive any blood.So now you also know why AB group is called Universal Acceptor.

So we understood why mismatching and matching occurs.Now we must know what are the consequences of a mismatch.If there is mismatch then antibody of recipient will attack the antigen of donor and the reaction is called agglutination.Due to agglutination a clot like substance will be formed which will affect the blood flow in the body leading to serious problems.So the blood groups has to be matched always.Now here i want to mention a point.If you have really understood this mechanism a very common doubt will arise.You will think, why is that the antibody in the donors blood not attacking the antigens in recipients blood.We know O group is a universal donor and O group has got both antibody in it.Then why is the antibody in donors O group blood not affecting recipients antigen.The answer is: The antibodies in donors blood gets diluted in the recipients blood that it wont cause much effect or agglutination.Here the quantity of donors blood is very less compared to recipients full amount of blood.So this is the case of ABO blood groups.

Rh factor

The Rh factor is the factor responsible for giving positive and negative to blood groups.In case of Rh factor also same type of interaction occurs.Rh antigen is also a protein present in the RBC and if the Rh protein is present the blood group is called Rh positive and if the protein is absent Rh negative.The antibody in this case is called anti-Rh antibody.This antibody is little different from a and b antibody.We know that a and b antibody are present in plasma according to the blood group.But anti-Rh antibody are not present in the plasma, instead it will be produced by the body if a Rh antigen enter the body.In short anti-Rh antibody are not pre prepared and kept in body.The remaining points are similar to ABO blood system.If an Rh positive blood enters an Rh negative person or recipient,there will be agglutination reaction.So Rh positive cannot be given to Rh negative blood or recipient.But a Rh negative blood can be given to Rh positive and Rh negative.So considering this case also we can now say that the actual Universal donor is O negative and actual universal acceptor is AB positive.A summary is shown in the picture.The red blood drops indicate that donor can donate the blood to recipient or its compatible.

Thus you have understood why blood matching is done and how blood groups are matched.Based on this blood transfusion is carried out.As i have said earlier, there are about 160 other groups.But ABO system is most important.Still considering those other groups, its always advised to get the blood of close relatives so that better matching take place.A new practise is also there in which, the patients own blood is taken and stored so that it can be used for any future needs.Here we get a perfect match, because it our own blood!