A, B, AB and O are blood types. Blood types are determined by antigens which are like receptors on your red blood cells. So..
A blood type - has ';A'; antigens
B blood type - has ';B'; antigens
AB blood type - has ';A'; and ';B'; antigens
O Blood type - doesn't have any antigens hence it is ';O'; meaning none
To preserve life one makes antibodies to foreign bodies and not to self. So if your blood type is an ';A'; you will not make an ';A'; antibody because it would be bad for your survival but you will make antibodies to what you don't have.
';A'; blood type - makes Anti ';B';
';B'; blood type - makes Anti ';A';
';AB'; blood type - doesn't make any antibodies because they have both of the possible antigens on their cells
';O'; blood type - makes Anti ';A';, Anti ';B'; and Anti ';AB'; because they don't have any antigens on their cells
Because ';O'; type blood doesn't have any antigens on their blood cells an ';O'; person's blood cells can be given to anybody. That is why they say that ';O'; blood group is the universal donor.
Group ';AB'; people have no antibodies to any blood group so they are the Univeral Recipients, they can have any type of blood transfused to them and they won't be harmed. Generally you give the blood group of the person as the first choice. ie. ';AB'; person should get ';AB'; blood but ';A'; or ';B'; or ';O'; would work in this case.
So if you are an ';A+'; that means you have ';A'; antigens on your red blood cells and the only blood cells that you can be transfused with are ';A'; and ';O';. You can't have ';B'; blood because you make an anti B antibody and you would destroy the cells. Same thing goes for ';AB'; blood, your Anti ';B'; will destroy the ';B'; part of the ';AB'; cells. The reason you could get ';O'; blood is because it doesn't have any antigens on the cells.
As for the negative and positive part of the blood that is another blood system called Rhesus which is much more involved than the ABO system. The most important antigen in the system is ';D';. If you have it, they say you are positive if you don't have it then it is noted at ';d'; and it is said that you are negative. As a rule they like to teach that ';+'; people should get positive blood, but really ';+'; or ';-'; is acceptable if necessary. But if you are ';-'; you are only supposed to get ';-'; blood because you could make an antibody which in the future would cause problems, especially for women because of pregancy. 85% of the population is ';+';How does this work?
If your 15, then I will answer this as simply as possible. Basically, A, B, AB and O are representative of different sugar molecules (like glucose) on sphingolipids (molecules on the outside of cell membranes of RBCs). The positive is an RH factor which is a protein on the outside of RBCs. Basically, if a person has any of these things on the outside of their cells, they will be recognized by people who don't. It's like if everyone wears a uniform to your school and someone shows up with a different uniform, they would stand out. So, someone with O-type will recognize blood that is A, B, or AB and attack those blood cells. A would recognize B, or AB and attack it. AB will be fine with anything because O doesn't have any extra things on the outside and it recognizes A and B. For RH, someone with a negative will recognize those who are positive and attack it. Worst case is that O- cannot receive from anyone because it recognizes any other blood type as different because it does not contain anything on the outside. AB+ can accept from anyone. I hope this helps. O- is a universal donor because, again, it has nothing on it so there is nothing to recognize by a person. It is essentially, naked.
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