What does Defects in the complement system cause?

Defective C2 and C4 molecules (involved in classical pathway of complement activation) develop systemic lupus erythematosus Deficiencies in the alternative complement pathway (i.e. Properdin) results in increased susceptibility to infection with piogenic bacteria* (possible cause, inability to form C3b which is required to opsonise bacteria) deficiencies in the complement components C5 to C9 suffer diseminated infections with Neisseria meningitidis and N. Gonorrhoeae.