Figure 1 Proteinase and proteinase inhibitor cascade showing the inter‐relationship between cysteine, serine and metalloproteinases. Individual proteinases can activate proteinases in another class and, at the same time, inactivate the relevant inhibitors. The net result is a cascade where the presence of several classes of proteinase can facilitate the end result of damage by an individual enzyme or in an additive way. MMP, matrix metalloproteinase; α₁‐AT, α₁‐antitrypsin; TIMP, tissue inhibitor of metalloproteinases; SLPI, secretory leucoproteinase inhibitor.