Complex sentence: C1-inhibitor (C1-inh, C1 esterase inhibitor) is a serine protease inhibitor (serpin) protein, the main function of which is the inhibition of the complement system to prevent spontaneous activation.
Split into simpler sentences: C1-inhibitor (C1-inh, C1 esterase inhibitor) is a protease inhibitor belonging to the serpin superfamily. Its main function is the inhibition of the complement system to prevent spontaneous activation.