In pulmonary alveolar proteinosis (PAP) the air sacs (alveoli) in your lungs do not work properly. They become filled with a grainy material which consists of waste from a substance called lung surfactant causing the patient to feel breathless.
When you breathe your lungs are filled with air, which contains a high concentration of oxygen. Normally, the oxygen is absorbed by the small blood vessels around the air sacs in the lungs and transferred by the blood to all organs in the body. But when you have PAP, the alveoli are filled with lung surfactant preventing the blood from absorbing the oxygen.
Lung surfactant is made up of lipids (fat) and proteins and is an important substance that coats the air sacs to prevent your lungs from collapsing. It also helps the alveoli stay inflated so that oxygen can be absorbed.
Your body continuously produces new active surfactant and in healthy lungs the old, inactivated surfactant is digested by immune cells called alveolar macrophages. They are the dustmen or garbage collectors of the body and they have a very important task of keeping the alveoli clean from inhaled particles and inactive surfactant material.
In PAP lungs, however, the macrophages fail to clean the alveoli from the waste material. Consequently, the waste material builds up gradually in your lungs and eventually fills the alveoli making it difficult for the oxygen in the air that you inhale to enter the blood.
When you have PAP, your body experiences a breakdown of immune control. The breakdown causes your immune system to produce high levels of antibodies that attack a protein in your own body. Antibodies are the body’s foot soldiers whose job is to identify and neutralise harmful bacteria and viruses. When they wrongly start attacking proteins in your body, your body experiences a breakdown in its defence system.
In PAP patients, the antibodies or foot soldiers attack an important protein called Granulocyte Macrophage Colony-Stimulating Factor (GM-CSF). The GM-CSF protein itself is part of your body’s host defence or immune system too.
In other words, your body starts attacking itself.
The GM-CSF protein is secreted by cells found in the lung alveoli (the terminal ends of the respiratory tree), where it serves to mature and stimulate the cells that defend your lungs against invading germs and regulate the breakdown of inactive surfactant. In healthy lungs, the GM-CSF protein activates the macrophages or dust men so they effectively can break down and get rid of the inactive lung surfactant they have ingested.
In PAP lungs, however, the GM-CSF protein does not work properly because it is under attack by the antibodies or foot soldiers who are present at high levels because of the breakdown of immune control. Consequently, the waste material gradually builds up and fills the alveoli in your lungs making it difficult for the oxygen you breathe to enter the blood.
When the lungs are filled with waste material from inactive surfactant, the process of transferring oxygen into the bloodstream is obstructed. This is why you may experience symptoms such as breathlessness, coughing and chest pain.