Question

Describe the differences between the classical and the lectin pathway of the complement system?

Answer 1

The main difference is that :-

- Classical pathway need antibody for its activation, but Lectin complement pathway doesn't depend on antibody for its activation. It depends on lectins for its activation.

- In classical pathway antigen-antibody complex is needed for activation. But in Lectin no antigen-antibody complex is needed.

- Lectins are proteins that recognise and bind to specific carbohydrate targets. Lectins that activates complement , binds to mannose residues present on the surface of microorganisms.

- After Lectin binds to the foreign surface of a pathogen , specific proteases like MASP1 and MASP2 bind to it and forms the C1 like active complex. Later it causes cleavage and activation of C4 and C2 to produce C5 convertase without the need for specific antibody binding.

- but in Classical pathway first C1 binds to the antigen-antibody complex . The recognition unit of C1 is C1q, which reacts with the Fc piece of bound IgM or IgG. C1q binding in the presence of calcium ions leads to sequential activation of C1r and s. Hence C1 activation complex ( C1qrs) is formed. Hereafter all the processes are same in both the pathway.

Hereunder I enclose comparison diagram of both the pathway:-

Hope you got your answer, thanks.