What is the interaction between Protein S and C4b-binding protein?

The affinity of protein S to C4BP is several hundred-fold increased by calcium ions. C4BP is an acute-phase reactant, and its concentration in plasma may increase as much as four-fold. This does not result in free PS levels approaching 0 however, because C4BP exists in 2 forms: under normal circumstances, about 10 – 15% of the C4BP circulating in plasma lacks the b-chain and hence does not bind PS. During an acute phase reaction, the C4BP which lacks the b-chain is increased and therefore no significant decrease of free PS occurs.