Circulating C1q levels in health and disease, more than just a biomarker

•C1q levels increase with age but are similar between men and women.•Decreased C1q levels have been observed in several autoimmune diseases and can be caused by defective production or increased consumption.•Increased C1q levels have been reported for infectious and inflammatory diseases and is suggested to inhibit the adaptive immune response.•C1q can protect against infections via the classical pathway, and maintain tolerance by regulating adaptive immunity, likely via C1q receptors.