Monovalent

Monovalent is an adjective used in chemistry, biology, and medicine to describe something with a valence or binding capacity of one. It comes from the Latin words for one and strength. In general, a monovalent species interacts in a single way—by forming one bond, carrying a single positive or negative charge, or engaging a single binding site.

In chemistry, a monovalent species has valence one. This includes ions such as H+, Na+, K+, and Cl−, which typically form a single ionic interaction or coordination. By contrast, divalent or multivalent species can form two or more bonds or carry higher charges. Monovalent terminology is often used to distinguish simple, single-charge ions from more complex, multivalent ions in reactions and materials.

In immunology and vaccinology, monovalent describes a preparation that targets a single antigenic determinant or strain. A monovalent vaccine contains antigen from one serotype or strain, whereas polyvalent vaccines include multiple strains or antigens. Monovalent formulations can enable focused immune responses and rapid protection against a specific pathogen, though they may not provide broad coverage.

In antibody engineering and protein science, monovalent can refer to molecules with one antigen-binding site. Most intact antibodies are bivalent, but fragments such as certain Fab fragments or engineered constructs can be monovalent, binding a single epitope. The term also appears in serology to describe antisera raised against a single antigen.

Overall, monovalent denotes one unit of binding or charge, distinguishing it from polyvalent or multivalent systems with multiple sites or charges.