Note:This is an oversimplified explanation and attempts only to give a broad overview of the immune system and different blood types.
A human body is made up of over 30 trillion (1 Trillion = 1012) cells, which are divided into about 200 cell-categories e.g. skin cells, stem cells, muscle cells, etc.
And because it is an attractive place (full of nutrition) for pathogens (virus, bacteria, etc.), the body is always at risk of invasion by these foreign objects.
To control this invasion, the body deploys over 20 billion (1 billion = 109) leukocytes (white blood cells – a key component in our blood) that roam around the body like security personnel.
But since the cell-population is huge, it could be difficult for leukocytes to identify any invaders (bacteria, virus, etc.) that move in secretly.
So, the body has given highly-visible tags to every cell that belongs to the body.
These tags or markers are called antigens and they read something like: “My name is skin cell and I am not an invader.”
The leukocytes keep monitoring all the cells in the body and the moment they sense anything whose tag doesn’t seem valid, they call the armed forces of the body (B-cells & T-cells) and give them the tag-details of this pathogen.
B-cells & T-cells produce customized antibodies (soldiers) that exactly match the pathogen tags and get attached to each pathogen like a key in the lock and kill all of them.
Now, 80% of the 30 trillion cells are Red Blood Cells (RBCs), and the body has sort of given it two tags – (Tag 1.) “My name is RBC and I am not an invader” & (Tag 2.) “I am a Type A RBC”.
While RBCs in everyone have the same Tag 1, different people have different 2nd Tags, i.e. some people have Type A RBC, some have Type B, some have both Type A & Type B (AB) and some don’t have the 2nd tag at all (this last group has the O blood group).
If we transfused Type A blood in someone who has the blood-group A, the leukocytes will see it as something that belongs and, therefore, no action will be taken.
However, if we transfused Type B in a Type A body, the leukocytes will see B as a foreign object and get the armed forces to create Anti-B antibodies that could attack it, thereby, making the blood clot (may cause death).
Because Type O blood doesn’t have the second tag (O basically means 0 [zero antigen tags]) and the first tag matches, the leukocytes have no “bad” tags to get antibodies against.
Basically, leukocytes can’t request any antibodies against RBCs and other than that there is no antigen tag against which antibodies need to be created.
That is why Type O is called a universal donor (it can donate to anybody) and Type AB is called universal recipient (it can accept blood from anybody), because the leukocytes will recognize A, B and not arrange for the attack.
