Antigens are usually found on germs such as infectious bacteria or viruses. The immune system reacts to neutralize these germs and prevent infections. Antigens are also found on harmless material such as mould spores. Farmer's Lung develops in two steps, as other allergic reactions do. The first time a person inhales a large amount of dust and spores from moldy hay, the immune system often responds by producing specific antibodies against the antigens.
Antibodies are chemicals that circulate in the blood and attack specific antigens.
Once the body has produced these antibodies, there is a chance that further exposure to moldy dust can generate a hypersensitive type of allergic reaction. This reaction is typical of Farmer's Lung. Certain types of white blood cells move into the lungs to attack the antigens. Then, other types of white blood cells move into the lungs, releasing chemicals and poisons that increase blood flow and kill lung tissue close to the antigens. This response causes swelling, inflammation and damage to lung tissue, resulting in noticeable breathing problems.