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.
What changes occur when the blood reaches the lungs? When the blood is pumped through the pulmonary artery to the  pulmonary capillaries The Oxygen diffuses across the  alveolar-capillary membrane and is absorbed by the hemoglâ¦obin in  the red blood cells.    That's the simple form.