complement
The complement system is found in blood plasma and tissue fluids, and is a reaction to an infection.
It is another part of nonspecific (innate) immune system that is not a cell.
The complement system consists of a series of about 20 proteins.
The 'official' description of the complement system is a 'cascading sequence of enzymes'.
It is cascading because when one enzyme is triggered, it does not just trigger off the next enzyme, but rather it triggers off several copies of the same enzyme.
It is sequential, because it does this in a particular sequence.
| REFLECTION
To understand this, I often think of my misspent youth which was spent amongst other places in the local billiard hall. One of the tricks we used to attempt was to line up the billiard balls in such a way that when you hit the first one, that ball hit the next two, each of which hit another two balls, and so on. The really clever ones (or the ones who had really misspent their youth) could then get them all into the pocket (I never could!). Or, to take another example, have you seen those 'domino pictures' where a room is filled up with dominoes which are set up in such a way so that when you knock the first one down, it sets up a chain reaction which ends up by all of them being knocked down and the end result is a pretty picture (always seems a pointless exercise to me, but never mind!). Both of these are examples of cascading sequences (or sequential cascades - which ever description turns you on). |
The complement system works like this:
When the first component of complement is activated, a complex cascade reaction is set in motion (i.e. the domino effect) in which the products of the reaction at each step in the sequence activates several copies of the next component in the cascade.
There are 3 pathways which initiate the complement system:
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the classical pathway |
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the alternative pathway |
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the mannan binding lectin (MBL) pathway |
The classical pathway is activated by both the invading micro-organism and the antibodies released in response to these micro-organisms.
The alternative pathway is triggered off by the products of micro-organisms, and does not need to wait for the production of antibodies.
The alternative and MBL pathways join the classical pathway at C3 ('C' just stands for Complement ) and the pathways then share the remaining complement sequences.
| REFLECTION
Just take a little time out to reflect on the 3 cascading sequences. What do you think might be the advantage of having three 'beginnings' to the complement system? |
You may have noticed that the numbering of the start of the classical pathway is rather peculiar, being C1, C4, C2, C3.
Don't worry, it's not that I cannot count, it is just simply that these complement factors were numbered in the order in which they were identified and C4 was identified before C2 and C3.
C1q, C1r, and C1s are known as the 'RECOGNITION UNIT' - these complement components 'recognize' that there is an infection.
C4, C2, and C4 are the ' ACTIVATION UNIT' - they start various things happening.
C5, C6, C7, C8, and C9 make up the 'MEMBRANE ATTACK UNIT' which actually attacks the cell wall of the micro-organism and kills it (this is known as CELL LYSIS).
They do this by puncturing the cell membrane and causing the cell to implode.
As well as working as a sequence towards cell lysis, the individual complement proteins also have other roles to play, for example C1s increases vascular permeability, as do some of the other complement factors, whilst others are involved in chemotaxis (the movement of cells and other inflammatory factors to the place where they are needed), and they also are involved in opsonisation.