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Arctium lappa is a perennial species of the Asteraceae family originally from Europe and Asia. Considered a weed species in the southern region of Brazil, it is popularly used as a natural anti-inflammatory. The complement system is an important component of the innate immune response. However, its exacerbated activation can lead to harmful conditions like autoimmune and inflammatory disorders. Plants that inhibit the activation of complement can be a promising tool in the treatment of inflammatory diseases. Here, we evaluated the effect of A. lappa leaves extracts on the activation of the classical and alternative pathways of complement system. Two extracts were obtained under supercritical conditions using scCO2 with ethanol as cosolvent, at 313.15K, 15 MPa (E1) and 25 MPa (E2). Classical and alternative activation were evaluated using complement fixation test. Different concentrations of A. lappa extracts E1 and E2 showed an inhibitory effect on both complement pathways, and heparin was used as control. The IC50 of E1, E2, and heparin were 28.26, 20.12 and 92.54 µg/mL for classical and 26.12, 27.70, 27.78 µg/mL for the alternative pathway. Results demonstrate that A. lappa is a promising complementary treatment for diseases associated with complement activation.