American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the most common source of antigens causing hypersensitivity pneumonitis?

1. Dust mites

2. Thermophilic actinomyces

3. Pollen

4. Animal dander

The correct answer is: Thermophilic actinomyces

Hypersensitivity pneumonitis, also known as allergic alveolitis, is an immune-mediated inflammatory response in the lungs resulting from the inhalation of organic antigens. The most common source of these antigens is related to specific environmental exposures, particularly those associated with organic dust containing thermophilic actinomyces. These are a type of bacteria commonly found in environments with decaying vegetation, such as farm settings, and can be a significant trigger for hypersensitivity pneumonitis in susceptible individuals. Thermophilic actinomyces are often found in moldy hay or compost, where they thrive and can produce antigens that provoke an immune response. Upon inhalation, these antigens stimulate an inflammatory response in the lung parenchyma of sensitized individuals, leading to symptoms of hypersensitivity pneumonitis. Other allergens, such as dust mites, pollen, or animal dander, do not commonly lead to this specific pulmonary condition. While these allergens can trigger different types of allergic reactions or respiratory issues (like asthma or allergic rhinitis), they are not the primary cause of hypersensitivity pneumonitis. The specificity of thermophilic actinomyces in particular environments highlights its role as the most prevalent antigen in cases of hypers