Human blood is grouped into four different types. These four different types are : A, B, AB, and O. As you might guess, AB is a combination of, well, A and B. Each letter refers to a kind of antigen (protein) on the surface of the red blood cells. Red blood cells and other formed elements only make up about 45% of blood, the other 55% is this watery, sticky substance called plasma. For example, a person with type A blood would have A-antigens on the surface of every last little one of their red blood cells.
Each blood type is also grouped by its Rhesus factor, or Rh factor. …
