What is the first component to be activated in the classical complement pathway?

In the classical complement pathway, the first component to be activated is C1. This pathway is initiated when an antigen-antibody complex forms, allowing the C1 protein complex to bind to the Fc region of the antibody. C1 is composed of three subcomponents: C1q, C1r, and C1s. Upon activation, C1q binds to the antibody, leading to a conformational change that activates C1r, which in turn activates C1s. The activation of C1 then triggers a cascade of reactions that ultimately result in the cleavage of C2 and C4, and the production of C3 convertase. This step is crucial as it sets in motion the entire complement activation process, which plays a significant role in immune responses, including opsonization, inflammation, and lysis of pathogens.