Why would there be no immune response to gangrene?

Infection. There is an immune reaction .Gangrene is an infection that severely affects blood flow, and tissue death. Once blood flow stops, there is tissue death, and necrosis.
Gngrene. Gangrene comes from dead tissue damaged from lack of proper blood supply or injury. Secondary bacterial infection can form in the dead tissue. There would be a marked inflammatory response. The immune system would try to stop the bacterial invasion into the healthy tissue but likely would be overwhelmed by the bacteria & toxins. Surgery may be used to debride the dead tissue.