Alpha-1 antitrypsin (AAT) deficiency is condition passed down through families in which the liver does not make enough of a protein that protects the lungs and liver from damage. The condition can lead to emphysema and liver disease.

Alpha-1 antitrypsin (AAT) is a type of protein called a "protease inhibitor." AAT normally works to protect the lungs from a naturally occuring destructive enzyme that is produced by the lungs in response to infections and toxins (such as cigarette smoke). Without enough AAT, these enzymes slowly destroy the lung tissue.

AAT deficiency is caused by a genetic defect that results in not enough AAT in the liver, lungs, and blood. Persons with this deficiency may also develop liver disease.

Studies show that AAT deficiency may be more common than once thought. Most adults with severe deficiency will develop emphysema, which often begins before age 40. Smoking can increase your risk. The condition is most common among Europeans and North Americans of European descent.