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What is (are) the role(s) of complement proteins in the immune response?
I. They attract white blood cells to the site of infection.
II. They help other cells avoid infection.
III. They cause pathogens to clump together, making the pathogens easier to identify and destroy.
IV. They form a pore in the plasma membrane of a pathogen, causing it to rupture.

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The correct answers are I and  IV

There are three main effects  of complement :

1. Lysis of cells such as bacteria and tumor cells.

2. Generation of mediators that participate in inflammation and attract neutrophils to the site of infection, an event called chemotaxis.

3. Opsonization i.e. enhancement of phagocytosis.

Lysis of pathogens by complement involves the insertion of  a special substance called the membrane attack complex into the cell membrane of the pathogen which disrupts or punctures the membrane as  a result of which water  and electrolytes enter the pathogen and it dies. 

Generation of mediators known as C5a and C567  complex attract neutrophils.