ELI5- What are blood groups? Why do they differ from person to person? Why can’t a person of one blood group receive blood from a person of a different blood group?

Question

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In: Biology

Answer 1

Your cells respond to each other by producing proteins. A significant part of the immune system works by detecting proteins that cells display on their surface like a kind of braille. Your immune cells literally creep up on your blood cells (and all other cells) and ‘feel’ this braille to see if the cell is doing what it is supposed to be doing. This is how they can detect foreign objects, bugs like bacteria, and things like cancerous cells (most people actually develop ‘technically cancer’ at multiple times in their lives and their immune system just deals with it without them ever noticing, ‘actual cancer’ is the result of several things going wrong at once!).

Your ‘blood group’ then is a kind of protein that blood cells display on their surface. I think why these exist is beyond ELI5 (to do with sorting of cells during development in the womb I think?), but basically it turns out that between humans these proteins are fairly similar between people, there are only two types! A and B, how easy is that!? You then also have some people who don’t express either of these, which we call O. Because your immune system uses these proteins to recognize that the blood cell is actually *you* and not something foreign, this is quite convenient as the system is dumb enough for us to trick it. So long as the immune system is feeling this same braille pattern on the blood cell, it can’t *actually* tell where that cell come from it just moves along.

So then why people fall into different groups is kind of a separate question to do with genetics that others have kind of already answered, to do with what blood group proteins your parents express.

You also have the ‘positive’ and ‘negative’ rhesus blood groups, which is the same principle as the A/B/O blood group types, but another protein braille pattern that’s being felt.