Figure 1. Macrophages express the components of the urokinase plasminogen activation system. Serine proteases activate pre-uPA (the uPA zymogen) to uPA, which then binds to uPAR, and efficiently activates plasminogen to the enzyme plasmin, localizing it to the plasma membrane. Plasmin is important to a number of functions including immune cell migration. By inhibiting uPA, serpinB2 can block plasminogen activation.