Hypersensitivity Pneumonitis Make an Appointment Find a Doctor Ask a Question Reviewed by Cecile S. Rose, MD, MPH (June 01, 2012) Hypersensitivity pneumonitis (HP) is an inflammation of the lungs caused by inhaling a foreign substance. These foreign substances can stimulate an immune reaction in the small airways and air sacs of the lung. This may lead to acute lung disease. Over time and with continued exposure to the causative substance, long-lasting (chronic) lung disease with permanent scarring can occur. Programs & Services Deployment-Related Lung Disease Center Clinical Trials For more than 100 years, National Jewish Health has been committed to finding new treatments and cures for diseases. Search our clinical trials.