Alpha 1 antitrypsin or a1-antitrypsin (A1AT) is a glycoprotein and generally known as serum trypsin inhibitor. The correct name, however, is alpha 1 proteinase inhibitor (A1PI) because it is a serine protease inhibitor (serpin) inhibiting a wide variety of proteases.  Alpha 1 antitrypsin protects tissues from enzymes of inflammatory cells, especially elastase, and is present in human blood at 1.5 - 3.5 gram/liter, but the concentration can rise manyfolds upon acute inflammation.  In its absence, elastase is free to break down elastin - which contributes to the elasticity of the lungs -- resulting in respiratory complications such as emphysema leading finally to COPD (chronic obstructive pulmonary disease). Alpha 1 antitrypsin is a 52 kDa serine protease inhibitor, and in medicine it is considered the most prominent serpin, given the fact that the words alpha 1 antitrypsin and protease inhibitor (Pi) are often used interchangeably.  Most serpins inactivate enzymes by binding to them covalently, requiring very high levels to perform their function. In the acute phase reaction, a further elevation is required to "limit" the damage caused by activated neutrophil granulocytes and their enzyme elastase, which breaks down the connective tissue fiber elastin. Like all serine protease inhibitors, Alpha 1 antitrypsin has a characteristic secondary structure of beta sheets and alpha helices. Mutations in these areas can lead to non-functional proteins which can polymerise and accumulate in the liver (infantile hepatic cirrhosis). Alpha 1 antitrypsin is an a1 proteinase inhibitor that is expressed in the liver and the intestine. It acts as an acute phase protein by inhibiting serine proteases (e.g. elastase, trypsin, or chymotrypsin) and inflammatory processes by forming reversible complexes. In faeces, a1 antitrypsin is found both free or conjugated to either trypsin or elastase. The higher concentration of proteinases in the faeces is caused by an increased permeability of the intestinal epithelial layer which in turn is set off by an abnormal array of the intestinal “tight junctions” that subsequently results in an enteral loss of plasma alpha 1 antitrypsin. Faecal loss of alpha 1 antitrypsin has now become an accepted parameter for the evaluation of an enteral protein loss. The ELISA is now an established method, especially for the diagnosis of M. Crohn. It could be established, that this marker is well suited for indicating changes of intestinal permeability in cases other than colitis or M. Crohn. Immunological processes in the intestinal mucous layer, that may be caused by food intolerance, also result in an increase of faecal alpha 1 antitrypsin concentrations.