THE IMMUNE SYSTEM NEW DEVELOPMENTS IN RESEARCH (RECOGNIZING INVADERS)

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RECOGNIZING INVADERS

Hello friends today we will continue from where we left last time.

Let me now tell you about complement proteins.
The complement system is made up of over 25 proteins and protein fragments, which assist the
ability of antibodies and phagocytic cells to clear pathogens from an organism.
They are normally produced by the liver cells and circulate as inactive precursors in the blood.
When infection occurs, microbial products may activate, proteases in the system, which cleave
specific proteins to release cytokines, and this amplifies a cascade of further cleavages.
The end-result of this activation cascade is massive amplification of the response and activation
of the membrane attack complex on the surface of the pathogen, which leads to killing of
microbes.
Here I show you an example of complement cascade activation by infection.
In this system, complement proteins are inactive until they are cleaved by a protease, which in
turn, converts the proteins into a protease.
For instance, binding of an antibody to a pathogen triggers the complement activation through
the so-called classical pathway.
And the antibody bound to the pathogen can activate C1, which activates C2 and C4, which
activates C3 and C5, which leads to the sequential activation of C6, C7, C8 and C9, which
finally results in the formation of the membrane-attack complex or MAC on the surface of the
pathogen.


The MAC in turn functions as a channel, allowing the passage of ions and small molecules,
which causes osmotic swelling and killing of the pathogen or infected cells.
Alternatively, when a complement component C3 binds to the microbial cell surface, this
activates a number of other complement proteins in sequence.
This is called an alternative pathway.
Some of the products induce blood vessel dilatation, histamine release from mast cells,
recruitment of phagocytes.
Some of them induce promotional microbial phagocytosis and killing locally, as you see here.
Now, in addition to the classical pathway and the alternative pathway, there is another
complements activation pathway - the lectin pathway.
Upon activation of complement proteins through these pathways, now, inflammatory leukocytes
are recruited to the site of infection, phagocytosis by the activated leukocytes is promoted, and
pathogens are killed at the site of infection.

So, you can see now how important the complement system is as a component of the innate
immune system.
This ends our recognizing invaders. Next we will enhance our knowledge about Innate immune system activation. 
Thank you for reading this.

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