Hello Tanuj,
Greetings!

A blood group (or a blood type) is a categorisation of blood, which is basically based on the presence and absence of antibodies and inherited antigenic substances, present on the surface of Red Blood Cells (RBCs). There are plenty of blood groups but according to an Austrian scientist, Karl Landsteiner, blood can be classified as either A, B, O or AB.

In this type of classification (commonly known as ABO grouping system ), the cell membrane of RBC can have antigen A, B, neither, or both. People with blood type A would have a A antigen whereas people with blood type B will possess B antigen. Similarly, people with AB blood have both antigens, while people with blood type O have neither antigen.

Our immune system works in a manner that in order to create balance, it naturally develops antibodies for the antigens it lacks . This means that people with blood type B have an antibody against blood type A, and vice versa . It is due to the presence of antibodies in the body that we cannot infuse blood from anybody. For example, if a person has A blood group, then he/she will develop antibodies against blood type B. If such a person is given blood type B, then there will be a malfunctioning as the antibodies are nor compatible with such blood group and the red blood cells will be destroyed, leading to further complications.

People with blood type AB have no antibodies whereas people with blood type O have antibodies for types A and B . Blood type O is considered as the universal donor because it has no A or B antigens on its RBC membranes . Though they have no antigens on their surface but have all the blood type antibodies in the  plasma. This makes it neutral and safe to mix with blood types A, B, and AB (because these have no antibodies to reject type O or react against it) .

It should be noticed that though O type red blood cells are considered universal donor and can safely be given to the majority of people, there are few exceptions for certain extremely rare blood types who must receive their own type such as Bombay or Rh null.

Apart from the ABO classification, blood is also categorised by Rhesus (Rh) factor , which refers to a specific red blood cell antigen in the blood. If your blood has the antigen, you're Rh positive. If your blood lacks that antigen, you're Rh negative . Since blood group O has no antigens, it is negative, making O -ve as a universal donor.

I hope you find this information useful.

In case of any doubts, feel free to raise more queries, we will be happy to assist you.

All the best for your future endeavours!