text: Humans have the following blood types along with their respective antigens and antibodies:\n\n    * Individuals with type A blood have red blood cells with antigen A on their surface and produce antibodies against antigen B in their blood serum. Therefore an A-negative person can only receive blood from another A-negative person or from an O-negative person.\n    * Individuals with type B blood have the opposite arrangement, antigen B on their cells and produce antibodies against antigen A in their serum. Therefore, a B-negative person can only receive blood from another B-negative person or from an O-negative person.\n    * Individuals with type AB blood have red blood cells with both antigens A and B and do not produce antibodies against either antigen in their serum. Therefore, a person with type AB-positive blood can safely receive any ABO type blood and is called a "universal receiver". However an AB-positive person cannot donate blood except to another AB-positive person.\n    * Individuals with type O blood have red blood cells with neither antigen but produce antibodies against both types of antigens. Therefore, a person with type O-negative blood can safely donate to a person with any ABO blood type and is called a "universal donor". However an O-negative person can only receive blood from another O-negative person.\n\nOverall, the O blood type is the most common blood type in the world, although in some areas, such as Sweden and Norway, the A group dominates. The A antigen is overall more common than the B antigen. Since the AB blood type requires the presence of both A and B antigens, the AB blood type is the rarest of the ABO blood types. There are known racial and geographic distributions of the ABO blood types. \n\nThe precise reason why people develop antibodies against an antigen they have never been exposed to is unknown. It is believed that some bacterial antigens are similar enough to the A and B glycoproteins, and that antibodies created against the bacteria will react to ABO-incompatible blood cells.\n\nApart from red blood cells, the ABO antigen is also expressed on the glycoprotein von Willebrand factor (vWF), which participates in hemostasis (control of bleeding). In fact, blood type O predisposes very slightly to bleeding, as vWF is degraded more rapidly. ABO antigens are also present in many other tissues such as liver, kidneys and lungs\n\nThe H antigen:\nThe A & B antigens are derived from a common precursor known as the H antigen. The H antigen is a glycosphingolipid (sphingolipid with carbohydrates bonded to the ceramide moiety) which is modified to produce the A and B antigens. In type O blood, it remains unchanged and consists of a chain of glucose, galactose, N-acetyl galactosamine, and fucose attached to the ceramide. Since it lacks N-acetylneuraminic acid (sialic acid) it is referred to as a globoside, not a ganglioside. Type A has an extra N-acetyl galactosamine bonded to the galactose near the end, while type B has a third galactose bonded to that near-end galactose.