How aHUS works?

Our body has its own defence mechanism against external pathogens (infections). The 'complement system' of the body is an important part of this defence mechanism. This system consists of a number of proteins, synthesized in the liver, which work in tandem to attack along with other mechanisms in the body and destroy these infectious pathogens, keeping the body safe from illnesses of various kinds. This, in essence, is a part of the immune system of the body.

It is however crucial that this complement system acts in a controlled manner. Otherwise, it would end up attacking the body's own cells. For this purpose, there are a number of regulatory proteins. also synthesized in the liver, that work to check the action of the proteins of the 'complement activation cascade' - the series of actions that actually attack the external pathogens. In fact, the importance of this checking mechanism can be gauged from the fact that there are as many regulatory proteins as there are complement activation proteins!

Another noteworthy aspect is that there are three such complement pathways - the classical pathway, the alternate pathway and the lectin pathway. Each of these pathways works in different scenarios, in slightly different ways.

Now, when the regulatory system of the alternate complement pathway malfunctions, the alternate pathway undergoes uncontrolled activation. The controlling system fails and the proteins in the alternate pathway begin to attack the body's own cells. Atypical HUS is the result of this uncontrolled activation of the alternate complement pathway. In this, the alternate complement pathway attacks the body's own blood cells and causes them to fragment. These fragments spread all over but the size of the kidney's cells are where there is a good fit in terms of the size of the fragments. So, these fragments of blood cells go and get lodged in the cells of the kidney gradually causing the kidney function to decline.