Answer :
Final answer:
All three complement pathways—classical, alternative, and lectin—use C3 convertase to cleave C3. The classical pathway is initiated by an antibody-antigen complex, the alternative by spontaneous activation, and the lectin pathway by acute-phase proteins binding to a pathogen.
Explanation:
The complement pathway that uses a C3 convertase to cleave C3 into C3a and C3b is a critical aspect of the immune system's defense mechanism. The classical pathway, alternative pathway, and lectin pathway each initiates the complement system in different ways, but all converge on the activation of C3 convertase, which cleaves C3 into C3a and C3b.
The classical pathway is initiated by the binding of C1 to an antigen-antibody complex on the surface of a pathogen. The alternative pathway is activated spontaneously, with C3 convertase forming on the surface of the pathogen without the need for antibodies. The lectin pathway, similar to the classical pathway, requires the binding of an acute-phase protein, such as mannose-binding lectin (MBL), to a pathogen.
In summary, while all three pathways lead to the activation of C3 convertase and the cleaving of C3, they are initiated by different mechanisms. The classical pathway relies on the presence of antibody bound to antigen, the alternative pathway does not require antibodies and is initiated spontaneously, and the lectin pathway is triggered by the binding of an acute-phase protein to a pathogen.
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