The '''C1q complex''' is a [[protein complex]] involved in the [[complement system]], which is part of the [[innate immune system]]. C1q is part of the [[C1-complex]].
C1q can bind to the [[Antibody|antibodiesAntibodies]] of the [[IgM]]adaptive andimmune [[IgGsystem]] (allcan butbind [[Immunoglobulin_G#Subclasses|IgG4antigen]]), when the antibody is bound toforming an [[antigen-antibody complex]]. When C1q binds to IgM or IgG complexed with antibody-antigen complexes, the [[C1-complex]] is activated. Activation of the C1-complex intitiates the [[classical complement pathway]] of the complement system. Only the antibodies [[IgM]] and all [[IgG]] subclasses except [[Immunoglobulin_G#Subclasses|IgG4]] are able to initiate the complement system.
