Complement is an integral component of immunity. Inherited deficiencies in the ability to activate complement increases susceptibility to bacterial infection and autoimmunity. Examples include: meningitis and terminal component deficiency; systemic lupus erythematosus-like illness and classical pathway deficiency. Inherited deficiencies in the ability to control complement activation result in damage to host tissue. Examples include: red cell lysis in paroxysmal nocturnal haemoglobinuria (PNH); glomerular inflammation in C3 glomerulopathy; glomerular thrombosis in atypical haemolytic uraemic syndrome (aHUS). Therapeutic inhibition of complement activation is achieved through antibody-mediated inhibition of C5 activation (eculizumab). Eculizumab is currently licenced for both aHUS and transfusion-dependent PNH. I will discuss how complement deficiency results in disease, our current treatment approaches and how these may apply to rheumatic disease.

Disclosure of Interest None Declared