Each blood type possesses a different antigen with its own special chemical structure. Your blood type is named for the blood type antigen you possess on your red blood cells.
Visualise the chemical structure of blood types as antennae of sorts, projecting outwards from the surface of our cells into deep space. These antennae are made from long chains of a repeating sugar called fucose, which by itself forms the simplest of the blood types, the O antigen of Blood Type O. The early discoverers of blood type called it "O" as a way to make us think of "zero" or "no real antigen". This antenna also serves as the base for the other blood types, A, B and AB.Blood Type A is formed when another sugar called N-acetyl galactosamine is added to the O antigen, or fucose. So, N-acetyl galactosamine plus fucose equals Blood Type A.
Blood Type B is also based on the O antigen, or fucose, but has a different sugar, named D-galactosamine, added on. So, fucose plus D-galactosamine equals Blood Type B.
Blood Type AB is based on the O antigen, fucose, plus the two sugars, N-acetyl galactosamine and D-galactosamine. So, fucose plus N-acetyl galactosamine plus D-galactosamine equals Blood Type AB.