Question

Arrange the following events of complement classical pathway in correct order:
A. Formation of membrane attack complex
B. Binding of C1 to antibody-antigen complex
C. Cleavage of C3
D. Formation of C5 convertase

• A. B $\rightarrow$ C $\rightarrow$ D $\rightarrow$ A
• B. C $\rightarrow$ B $\rightarrow$ D $\rightarrow$ A
• C. B $\rightarrow$ D $\rightarrow$ C $\rightarrow$ A
• D. D $\rightarrow$ B $\rightarrow$ C $\rightarrow$ A

Hint:

Remember the general flow of complement pathways: Initiation $\rightarrow$ C3 convertase $\rightarrow$ C5 convertase $\rightarrow$ MAC. The defining starting point for the \textit{classical} pathway is always antibody binding (C1 complex activation).

Answer 1

The Correct Option is A

The classical pathway of the complement system is an essential part of the innate immune response, involving a sequence of protein activation leading to the destruction of pathogens. The correct order of events in the classical pathway is as follows: 

1. First, Binding of C1 to the antibody-antigen complex occurs. This initial step involves the recognition of antibodies bound to the antigen, typically IgG or IgM, by the C1 complex. This action initiates the cascade of reactions in the classical pathway.
2. Next, there is the Cleavage of C3. This is a pivotal step as it leads to the generation of C3b, a potent opsonin that enhances phagocytosis and aids in the formation of the C5 convertase.
3. Following C3 cleavage, Formation of C5 convertase happens. The C5 convertase complex facilitates the cleavage of the C5 component, leading to the production of C5a, an important inflammatory mediator, and C5b, which initiates the formation of the membrane attack complex.
4. Finally, the pathway concludes with the Formation of the membrane attack complex (MAC). MAC creates a pore in the pathogen's membrane, leading to lysis and destruction. This is the ultimate goal of the complement cascade, resulting in the elimination of invading microorganisms.

Putting the steps together based on the events mentioned:

Event	Step in Complement Pathway
B. Binding of C1 to antibody-antigen complex	First Step
C. Cleavage of C3	Second Step
D. Formation of C5 convertase	Third Step
A. Formation of membrane attack complex	Final Step

Thus, the correct sequence of events for the classical pathway is B → C → D → A, which matches the provided correct answer option.

Understanding this sequence is crucial for comprehending how the immune system effectively identifies, targets, and eradicates foreign pathogens using the complement system.