valence /vāl″əns/ [L, valere, to be strong] , 1. (in chemistry) a numeric expression of the capability of an element to combine chemically with atoms of hydrogen or their equivalent. An element is considered monovalent (or univalent) if each of its atoms can react with only one hydrogen atom or its equivalent, divalent (or bivalent) if each atom can react with two hydrogen or equivalent atoms, trivalent (or tervalent) if each atom can react with three hydrogen atoms, and polyvalent (or multivalent) if each atom can react with many hydrogen atoms. 2. (in immunology) an expression of the number of antigen-binding sites for one molecule of any given antibody or the number of antibody-binding sites for any given antigen. Most antibody molecules, and those belonging to the IgG, IgA, and IgE immunoglobulin classes, have two antigen-binding sites. Most large antigen molecules are multivalent.
