Blood is made up of red blood cells, white blood cells and platelets floating in a thick liquid called plasma. Plasma has many different giant molecules called proteins floating in it. Some of these proteins are known as antibodies and have a special function. These antibodies protect us from invading germs or foreign objects that get into our system.
Red blood cells have proteins that protrude from their surface. A red blood cells from an individual can have either an "A", "B", both "A" and "B" or neither "A" or "B". Antibodies known as anti-B cause clumping of cells with a "B" protein and anti-A antibodies cause cells with the "A" protein to clump. This is why we have to be careful when giving blood transfusions to people.
