Complement System

What is a complement system in innate immunity?

The complement system is part of the body's immune system and consists of a series of protein molecules which activate each other in a sequence known as a cascade. Complement system proteins are present in the blood stream and in the fluid that surrounds body tissues. When a pathogen such as a harmful microorganism enters the body, the complement system is triggered, and the proteins activate one another along one of three pathways

What are the three pathways that make up the complement system? How are they activated?

These are known as the classical complement pathway, the alternative complement pathway and the lectin pathway. Complement system activity coats pathogens so they are more easily targeted by the body's immune cells, and actively destroys those which are already attached to antibodies.

The classical pathway

In the classical pathway, part of the first complement protein, known as C1, binds to the antibody attached to an antigen on the surface of a bacterium. This binding activates another part of C1, which becomes an enzyme capable of splitting in half the complement proteins known as C2 and C4. The active part of C4 then binds to the bacterial surface and the active part of C2 attaches to it. This combination of parts of C2 and C4 also acts as an enzyme, which breaks up the next complement protein, C3. It is activated by the Fc portion of an immunoglobulin in an antigen complex.

The alternative complement pathway

This pathway is activated by the spontaneous hydrolysis of C3, which is abundant in the blood plasma to form C3(H2O). The hydrolysis changes the structure of C3 and increases its reactivity allowing for the binding of the serine protease factor B, which in turn is cleaved by the serine protease factor D to form Ba and Bb.

The Lectin pathway

This pathway is identical to the classical pathway except for the components involved in the initiation of the complement cascade. Lectin pathway's initiation is done by an opsonin, mannose binding lectin or ficolins. Mannose binding lectin binds to mannose residues on the pathogen surface which in turn activates the associated serine protease a MASP1 and MASP2.

What is membrane attack complex?

Membrane attack complex

This is an abnormal activation of the complement portion of the blood that forms a cascade reaction and brings blood proteins together, binds them to the cell wall, and then inserts them through the membrane. MAC allows water, ions and other small molecules to move freely into and out of a cell, and quickly results in cell death.

What is a cell Lysis?

Cell Lysis

Cell lysis is a process in which a cell is broken down or destroyed as a result of some external force or condition. Lysis can happen through natural means, such as viral infections, or through artificial means for research purposes. The resulting substance that contains the previous contents of the destroyed cell is referred to as the lysate; a substance that causes lysis, particularly an antibody, is called a lysin.

SOURCES

• Colledge, H., & Bailey, H. (n.d.). What is the complement system. Retrieved October 6, 2015.
• Janeway, C. (n.d.). The complement system and innate immunity. Retrieved October 6, 2015.
• Membrane attack complex,(n.d.). Retrieved October 6, 2015.
• Cell Lysis,(n.d.). Retrieved October 6,2015.