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→Classical Pathway
[[Image:LH Complement pathway.png|thumb|left|250px|'''Classical and alternative pathways''']]
[[Image:LH Complement pathway.png|thumb|left|250px|'''Classical and alternative pathways''']]
===Classical Pathway===
===Classical Pathway===
[[Image:Complement Classical Pathway.png|thumb|right|250px|'''Classical pathway in detail''']]
[[Image:Complement Classical Pathway.png|thumb|right|250px|'''Classical pathway in detail''' - R.J.Francis, RVC 2012]]
This pathway of the complement system is triggered by the binding of antibody to antigen. The only subclasses that can activate complement this way are the
This pathway of the complement system is triggered by the binding of antibody to antigen. The only subclasses that can activate complement this way are the
[[IgM]] and certain [[IgG]]. The first step of complement activation is the binding of C1 to two Fc regions of the antibodies (IgM is such a strong activator of complement as it contains '''five''' Fc regions, while IgG contains '''one'''). C1 is actually a complex of C1q, C1r and C1s. C1q looks like a bunch of 6 tulips with each "flower" consisting of a globular protein head and a collagen "stem". C1q is required to trigger the cascade because two of it is the globular heads binds the two Fc regions. C1r and C1s then become activated when the C1q heads are antibody-bound. When they are activated they form the enzyme '''C1 esterase''' (it is a serine protease).
[[IgM]] and certain [[IgG]]. The first step of complement activation is the binding of C1 to two Fc regions of the antibodies (IgM is such a strong activator of complement as it contains '''five''' Fc regions, while IgG contains '''one'''). C1 is actually a complex of C1q, C1r and C1s. C1q looks like a bunch of 6 tulips with each "flower" consisting of a globular protein head and a collagen "stem". C1q is required to trigger the cascade because two of it is the globular heads binds the two Fc regions. C1r and C1s then become activated when the C1q heads are antibody-bound. When they are activated they form the enzyme '''C1 esterase''' (it is a serine protease).