The results showed that several complement components, including classical pathway factors (C1q, C2), lectin pathway factors (C4, C4b, MBL), alternative pathway factors (complement factor B), C3 and its cleavage product C3a and C3b, C5 and its cleavage product C5a, complement factor H, and complement factor I were increased in the pleural fluid of LCP group, while complement factor D was decreased compared with the HP group (Figure 5A-G). Here, C2 is linked to Legg-Calve-Perthes disease.