What is (are) Hypersensitivity Pneumonitis ? 

Hypersensitivity pneumonitis (noo-mo-NI-tis), or HP, is a disease in which the lungs become inflamed from breathing in foreign substances, such as molds, dusts, and chemicals. These substances also are known as antigens (AN-tih-jens).
                
People are exposed to antigens at home, while at work, and in other settings. However, most people who breathe in these substances don't develop HP.
                
Overview
                
To understand HP, it helps to understand how the lungs work. When you breathe, air passes through your nose and mouth into your windpipe. The air then travels to your lungs' air sacs. These sacs are called alveoli (al-VEE-uhl-eye).
                
Small blood vessels called capillaries run through the walls of the air sacs. When air reaches the air sacs, the oxygen in the air passes through the air sac walls into the blood in the capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.
                
In HP, the air sacs become inflamed and may fill with fluid. This makes it harder for oxygen to pass through the air sacs and into the bloodstream.
                
The two main types of HP are acute (short-term) and chronic (ongoing). Both types can develop as a result of repeatedly breathing in an antigen.
                
Over time, your lungs can become sensitive to that antigen. If this happens, they'll become inflamed, which can lead to symptoms and may even cause long-term lung damage.
                
With acute HP, symptoms usually occur within 29 hours of exposure to an antigen you're sensitive to. Acute HP can cause chills, body aches, coughing, and chest tightness. After hours or days of no contact with the antigen, symptoms usually go away.
                
If acute HP isn't found and treated early, chronic HP may develop. Symptoms of chronic HP occur slowly, over months. Chronic HP can cause a worsening cough, shortness of breath with physical activity, fatigue (tiredness), and weight loss. Severe HP may cause clubbing (a widening and rounding of the tips of the fingers or toes).
                
With chronic HP, symptoms may continue and/or worsen, even after avoiding the antigen. Sometimes, chronic HP can cause long-term lung damage, such as pulmonary fibrosis (PULL-mun-ary fi-BRO-sis). This is a condition in which tissue deep in your lungs becomes scarred over time.
                
Outlook
                
Avoiding or reducing your contact with antigens can help prevent and treat HP. For example, cleaning heating and ventilation filters can help reduce your contact with mold. Wetting compost prior to handling it can reduce contact with harmful dust.
                
If HP is caught early, avoiding the antigen that caused it may be the only treatment you need. If you have chronic HP, your doctor may prescribe medicines to reduce lung inflammation.
                
Researchers continue to study why some people develop HP after being exposed to antigens, while others don't. They're also looking for better ways to quickly pinpoint which antigens are causing HP in people who are believed to have the disease.