Blood clotting cascade

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A number of proteins are involved in the series of events that lead to vertebrate blood clotting.

Michael Behe describes it thus:

When an animal is cut, a protein called Hagemann factor (XII) sticks to the surface of cells near the wound. Bound Hagemann factor is then cleaved by a protein called HMK to yield activated Hagemann factor. Immediately the activated Hagemann factor converts another protein, called prekallikrein, to its active form, kallikrein.[1]

Behe, who is a supporter of intelligent design, argues that all of these components must be in place before the system will work:

...none of the cascade proteins are used for anything except controlling the formation of a clot. Yet in the absence of any of the components, blood does not clot, and the system fails.[2]

Although, as Behe points out, Factor XII initiates the "cascade of proteins" in vertebrates, dolphins lack Factor XII [3] and yet their blood is known to clot. Similarly, molluscs such as snails have only four components in their blood clotting cascade, with a single protease step. This appears to contradict the notion of irreducible complexity in regard to this process.


  1. Behe, M. 1996. Darwin's Black Box. New York: The Free Press
  2. Behe, M. 1996. Darwin's Black Box, p.86. New York: The Free Press
  3. Robinson, A. J., M. Kropatkin, and P. M. Aggeler, 1969. Hagemann Factor (Factor XII) Deficiency in Marine Mammals. Science 166: 1420-1422