Complement Factor H (CFH) is an important inhibitor of the alternate complement pathway in Bruch's membrane (BM), located between the choriocapillaris and the retinal pigment epithelium. Furthermore dysfunction of its activity as occurs with certain polymorphisms is associated with an increased risk of age related macular degeneration (AMD). The retina is a site of high generation of reactive oxygen species (ROS) and dysfunction of redox homeostasis in this milieu also contributes to AMD pathogenesis. In this study we wanted to explore if CFH exists in distinct redox forms and whether these species have unique protective biological functions. CFH can be reduced by the naturally occurring thioredoxin − 1 in CFH domains 1–4, 17–20. We found a duality of function between the oxidised and reduced forms of CFH. The oxidised form was more efficient in binding to C3b and lipid peroxidation by-products that are known to accumulate in the retinae and activate the alternate complement pathway. Oxidised CFH enhances Factor I mediated cleavage of C3 and C3b whereas the reduced form loses this activity. In the setting of oxidative stress (hydrogen peroxide)-mediated death of human retinal pigment epithelial cells as can occur in AMD, the free thiol form of CFH offers a protective function compared to the oxidised form. We found for the first time using a novel ELISA system we have developed for free thiol CFH, that both redox forms of CFH are found in the human plasma. Furthermore there is a distinct ratio of these redox forms in plasma depending if an individual has early or late AMD, with individuals with early AMD having higher levels of the free thiol form compared to late AMD. Display omitted •CFH exists in an oxidised and reduced (free thiol) form in human plasma.•The oxidised form of CFH is more efficient in binding C3b.•The oxidised form of CFH enhances Factor I cleavage of C3 and C3b.•The free thiol form of CFH protects against free radical cell injury.•Reduced and oxidised CFH protect against early and late AMD in a reciprocal manner.