noun, plural: opsonins
An opsonin refers to any substance that enhances phagocytosis. An example of a natural opsonin is certain antibodies. Antibodies are glycoproteins produced by B cells. Their main function is to protect the body from foreign agents (called antigens) through an immune response against the antigens. There are different types of antibodies. They may be grouped based on their mode of action or by the reactions produced with the corresponding antigens. Some of these groups are agglutinins, bacteriolysins, haemolysins, opsonins, and precipitins. Antibodies that act as opsonins bind to the surface of an antigen and in doing so result in the phagocytosis of the antigen by a phagocyte, such as a macrophage.
Opsonins are essential in immunological protection of the host against pathogenic bacteria. Opsonins in the surface of the antigen serve as "flags" to indicate that the antigen needs to be destroyed by the action of the immune system. Opsonins also act on dead cells, marking them up for recycling.
The process wherein a substance is chemically modified to become delicious to a phagocyte is referred to as opsonization.
Word origin: Greek opsōneîn ("to prepare for eating")