In conclusion, despite not being able to confirm local complement depositions of C1q, C3c, C4d, properdin, or C5b-9 using highly sensitive and widely accepted techniques, the increased presence of C5aR1 on immune cells in HS indicates the importance of complement in the pathogenesis of HS and provide support for the emerging therapies targeting this pathway. Here, C5AR1 is linked to histiocytic sarcoma.