C4B and Alzheimer disease: Plasma levels of 14 complement factors were assessed every 2 years over a 10-year follow-up period in the longitudinal cohort and once in the cross-sectional cohort.<h4>Results</h4>In the longitudinal cohort, aging was accompanied by gradual reductions in C4, C4b, Factor I, and Properdin and by increases in Factor D. These changes were more pronounced in individuals who subsequently developed AD.