Not only immunoglobulins, C-reactive protein too.

The purpose of this letter is to expand a discussion, published recently in Molecular Immunology, on the post-secretion gain of function by immunoglobulins. It was reported that some circulating IgG molecules in all healthy individuals acquire novel antigen-binding specificity after exposure to conditions that change protein conformation, such as acidic pH. Another protein, C-reactive protein, also acquires novel ligand-binding specificity post-secretion after a switch from its native pentameric conformation to non-native pentameric conformation. Thus, that the functions of a protein depend upon its alternate structural states, and therefore on the surrounding milieu, is a more general phenomenon for some ancient molecules of the immune system than previously thought.