Serpins are a group of proteins with similar structures that were first identified as a set of proteins able to inhibit proteases. The acronym serpin was originally coined because many serpins inhibit chymotrypsin-like serine proteases (serine protease inhibitors).In humans, extracellular serpin-enzyme complexes are rapidly cleared from circulation.

In mammals, one mechanism by which this occurs is via the low-density lipoprotein receptor-related protein (LRP receptor), which binds to inhibitory complexes made by antithrombin, PA1-1, and neuroserpin, causing uptake and subsequent signaling events.