SerpinB2 or plasminogen activator inhibitor type 2 (PAI-2) is widely described as an inhibitor of extracellular urokinase plasminogen activator. However, the evidence that this represents the physiological role of SerpinB2 is not overly compelling. SerpinB2 is substantially up-regulated under multiple inflammatory conditions, and dysregulated expression and polymorphisms are associated with several human inflammatory diseases. A bewildering array of diverse functions and activities have been attributed to SerpinB2, but despite 蹰0 publications in the field, no coherent view of what SerpinB2 does in vivo has emerged. Although SerpinB2 is abundantly expressed by activated macrophages and a range of other haematopoietic and non-haematopoietic cells, SerpinB2(-/-) mice have surprisingly few phenotypes. However, SerpinB2(-/-) mice were recently shown to generate increased Th1 responses, suggesting that at least one function of SerpinB2 is sculpting of the adaptive immune response.