Blood group antigens are carbohydrates that are attached to proteins or lipids. An antigen is a substance foreign to the body that causes an immune response. An immune response occurs when antibodies, which are proteins in your immune system, are summoned to attack an antigen.
Blood that reaches the lungs travels throughout a network of small blood vessels, where oxygen moves into the blood and carbon dioxide moves out of the blood, according to the National Heart, Lung and Blood Institute.
Oxygen passes from the air into the blood in the lungs. More  specifically, the lung branches out like a tree, ultimately leading  air into tiny sacs called alveoli. Their m â¦ embrane separates blood  from air, but this membrane is thin enough for oxygen and carbon  dioxide to simply pass through.