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.
Your alveoli are surrounded by minute blood vessels, as this is where gases diffuse from your lungs into your blood and from your blood into your lungs. Oxygen passes from your alveoli into your blood and carbon dioxide, which is produced when your cells break down nutrients, passes from your blood into your alveoli.