Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Microscopic polyangiitis ?
    ### Response:
    What causes microscopic polyangiitis (MPA)? The cause of MPA is unknown. It is not contagious, does not usually run in families, and is not a form of cancer. The immune system is thought to play a critical role in the development of MPA.  It is thought that the immune system becomes overactive and causes blood vessel and tissue inflammation, which leads to organ damage.
    