This reaction is mediated by immune (Ag-Ab) complexes which promote tissue damage primarily through complement activation (alternate pathway). C3b as an opsonin attracts neutrophils, which then release lysosomal enzymes. C5a as a chemoattractant brings in neutrophils. Serum complement is reduced as it is used up in this process.
In the diagram above, antigen-antibody complexes are circulating and becoming trapped beneath the basement membrane of a small blood vessel, setting off the complement cascade and generating components that attract PMN's to generate an ongoing inflammatory response.
Local immune complex disease: Also called an "Arthus" reaction, it occurs with local injection of the antigen and leads to focal vasculitis. This kind of immune reaction also plays a role in the development of hypersensitivity pneumonitis (so-called "farmer's lung").