Question

Briefly describe the three alternative complement pathways.

Answer 1

Ans-The complement system is a part of the immune system, consists of a series of proteins that interact with one another in a highly regulated manner, in order to eliminate pathogens. It helps antibodies and phagocytic cells to clear pathogens and damaged cells; promote inflammation and attack pathogen’s plasma membrane. Proteins that take part in the complement system are called complements that collectively work as a biological cascade

Complements are soluble proteins and glycoproteins mostly produced by hepatocytes. More than 20 types of complements are present in serum, found circulating normally in human body in inactive forms (called as zymogens or proenzymes). Complement activation is triggered by an antibody when it is bound to the antigen. It can also be triggered by some components of innate immunity. Thus the complement system works in both innate and acquired immunity.

Complements are mainly denoted by the capital letter C with numbers; like, C1, C2, C3, and so on.

The complement activation occurs via three pathways; which are:

Classical pathway,Alternative pathway ,Lytic pathway (or mannose binding lectin pathway)

Alternative Pathway

This pathway involves various factors, B, D, H & I, which interact with each other, and with C3b, to form a C3 convertase, C3bBb, that can activate more C3, hence the pathway is sometimes called ‘the amplification loop’. Activation of the loop is promoted in the presence of bacterial and fungal cell walls, but is inhibited by molecules on the surface of normal mammalian cells.

Mannose-binding Lectin Pathway

This pathway is activated by the binding of mannose-binding lectin (MBL) to mannose residues on the pathogen surface. This in turn activates the MBL-associated serine proteases, MASP-1 and MASP-2, which activate C4 and C2, to form the C3 convertase, C4b2a.

Lytic Pathway

This pathway is initiated by the splitting of C5, and attachment of C5b to a target. C6, C7, C8 and C9 unite with C5b, and this membrane-attack complex (MAC), when inserted into the outer membrane of some bacteria, can contribute to their death by lysis. Red cells which have antibody bound to the cell surface can also activate the classical and lytic pathways, and become susceptible to lysis.