Alpha-1 antitrypsin deficiency (AATD) is a hereditary condition caused by one of many mutations in the SERPINA1 gene, resulting in reduced levels or absence of the alpha-1 antitrypsin (AAT) protein. AAT is produced in the liver and plays a vital role in protecting lung tissue from damage by neutrophil elastase, an enzyme released during inflammation (as a response to smoking tobacco or cannabis). 

When AAT is absent or dysfunctional, the balance between the proteases and their inhibitors is disturbed. Over time, this predisposes individuals to premature emphysema and, in some cases, liver disease due to the accumulation of misfolded protein in hepatocytes. Although AATD is one of the most common inherited metabolic disorders in northern Europe (affecting around 1 in 2000–5000 people), it remains underdiagnosed. 

Fewer than 10% of affected individuals are thought to have been identified, partly because many individuals may not have symptoms, or they overlap with common conditions such as COPD and asthma.