Complement can damage host tissue when overactivated. Evidence of complement self damage exists for Alzheimer disease (AD), age-related macular degeneration, type 1 diabetes mellitus (T1DM), and Parkinson disease (PD). Known complement activators include A beta, found in AD, and IgG found in T1DM. We compared their complement activating ability in vitro with those of islet amyloid polypeptide (IAPP), which aggregates in the pancreas of T2DM, and alpha-synuclein (alpha-Syn), which aggregates in PD. We found that IAPP and the alternatively spliced alpha-Syn 112 form, but not full-length alpha-Syn 140, activated complement in vitro. Complement activation may contribute to death of insulin-secreting cells in T2DM or to neuronal death in Parkinson disease (PD) and related synucleinopathies where alpha-Syn 112 occurs. This suggests the possibility of anti-inflammatory treatment in these pathologies. It also suggests that blockers of complement activation may be an appropriate therapeutic target for a range of age-related degenerative diseases. (c) 2007 Elsevier Inc. All rights reserved.