The classical complement pathway differs from the alternative pathway in that it requires a trigger in the form of an antigen-Ab complex. Only two antibodies can fix complement—IgG and IgM. The classical pathway is shown in Figure 5.3. The small fragments that are cleaved from the proenzymes have activities such as chemotactic stimulation and anaphylaxis. The bar over the names of some components of the pathway denotes an active complex. Note that the classical pathway does not operate with C1 to C9 in sequence. Rather, the sequence is C1, C4, C2, C3, C5, C6, C7, C8, and C9.
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