Serum MASP-1 in complex with MBL activates endothelial cells.

•Serum derived MBL–MASPs complexes activate endothelial cells.•Among the components of the complexes only MASP-1 can trigger response in cells.•MBL, MASP-2, MASP-3 and the N-terminal fragments are ineffective.•The proteolytic activity of MASP-1 is essential to trigger Ca2+ signal.•MASP-1 plays a central role in the early innate immune response.