In: Biology
Which compliment pathway(s) results in the production of MAC?
Which compliment pathway(s) involves antibodies despite being part of the innate immune system?
(multiple answers from the choices below are allowed, if applicable)
A). Manose-binding Lectin Pathway
B). Classic Pathway
C). Alternative Pathway
Question 1
All 3 complement pathway-classical, alternate and mannose binding lectin pathway results in formation of membrane attack complex. The 3 pathways differ from each other till the formation of C3 convertase.
Classical pathway - Begins when C1q binds with the Fc portion of the Ig bound to the antigen. Following this C1r and C1q is activated. Activated C1s acts as an esterase (C1 esterase) which cleaves C4 to C4a and C4b. C4b binds to C1. Now this C14b cleaves C2 to C2a and C2b. C2a binds to C14b to form C14b2a. This is the C3 convertase.
Alternate pathway - Initiated by various cell surface components of pathogens. C3b binds to the cell surface components. Factor B then bind to C3b. Now Factor D cleaves Factor B to Ba and Bb. Factor Bb binds to C3b to form C3bBb. This is the C3 convertase.
Mannose binding lectin pathway - Initiated by the mannose binding lectin proteins in the host cell that binds to mannose residues on microbial surface. Following this, another host protein called mannose binding lectin associated serine protease (MASP) binds to MBL. This will cleave C4 to C4a and C4b. C4b binds to MBL - MASP complex. This cleaves C2 to C2a and C2b. C2b binds to the complex. Hence MBL - MASP C4b2a is the C3 convertase.
The subsequent steps are same for 3 pathways.
The C3 convertase cleaves C3 into C3a and C3b. C3b binds to the C3 convertase to form C5 convertase. Sequentially C5b678 is formed which polymerises C9 into a tubular channel of 10nm diameter. Penetration of C9 forms pores in the target cell membrane.Water and ions enter the cell leading to cell swelling and lysis. C5b6789 is called membrane attack complex.
Question 2
Classical pathway is triggered by formation of antigen - antibody complex. It is a part of acquired immunity
Alternate pathway is triggered by endotoxin, teichoic acid, yeast cells, virus infected cells and also human immunoglobulin IgA and IgD. It is a part of innate immunity.
Mannose binding lectin pathway is triggered by Mannose carbohydrate residues on microbial surface. It is a part of innate immunity