A cascade reaction is formed and the end product is blood clotting. The entire cascade is the system while everything else are the parts required.
In modern "IC" systems removing one part may prevent the whole from working. But much of the problem with IC systems is that their proponents analyze them as modern intact systems without considering possible steps in the development of these systems, or even looking at lower organisms.
It seems there are some invertebrates that manage clotting reactions without having any dedicated clotting system at all:
Well, protein-rich plasma flows into an unfamiliar environment, and sticky white cells quickly "glom" up against the fibers of the extracellular matrix. Tissue proteases, quite accidentally, are now exposed to a new range of proteins, and they cut many of them to pieces. The solubility of these new fragments vary. Some are more soluble than the plasma proteins from which they were trimmed, but many are much less soluble. The result is that clumps of newly-insoluble protein fragments begin to assumulate at the tissue-plasma interface, helping to seal the break and forming a very primitive clot.
Taken from
The Evolution of Vertebrate Blood Clotting, an on-line document that gives some possibilities on how the modern human clotting cascade could have evolved from a single protein.
A counter question to keep in mind is this: Why would a designer make such a complex system for blood clotting when a simpler one would do?
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