Opsonins are extracellular proteins that, when bound to substances or cells, induce
phagocytes
Phagocytes are cells that protect the body by ingesting harmful foreign particles, bacteria, and dead or dying cells. Their name comes from the Greek ', "to eat" or "devour", and "-cyte", the suffix in biology denoting "cell", from the Greek ...
to phagocytose the substances or cells with the opsonins bound.
Thus, opsonins act as tags to label things in the body that should be phagocytosed (i.e. eaten) by phagocytes (cells that specialise in
phagocytosis
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis i ...
, i.e. cellular eating).
Different types of things ("targets") can be tagged by opsonins for phagocytosis, including: pathogens (such as bacteria), cancer cells, aged cells, dead or dying cells (such as apoptotic cells), excess
synapses
In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell.
Synapses are essential to the transmission of nervous impulses from ...
, or protein aggregates (such as
amyloid plaques
Amyloid plaques (also known as neuritic plaques, amyloid beta plaques or senile plaques) are extracellular deposits of the amyloid beta (Aβ) protein mainly in the grey matter of the brain. Degeneration (medical), Degenerative neuronal elements a ...
). Opsonins help clear pathogens, as well as dead, dying and diseased cells.
Opsonins were discovered and named "opsonins" in 1904 by Wright and Douglas, who found that incubating bacteria with blood plasma enabled phagocytes to phagocytose (and thereby destroy) the bacteria. They concluded that: “We have here conclusive proof that the blood fluids modify the bacteria in a manner which renders them a ready prey to the phagocytes. We may speak of this as an “opsonic” effect (opsono - I cater for; I prepare victuals for), and we may employ the term “opsonins” to designate the elements in the blood fluids which produce this effect.”
Subsequent research found two main types of opsonin in blood that opsonised bacteria:
complement
A complement is something that completes something else.
Complement may refer specifically to:
The arts
* Complement (music), an interval that, when added to another, spans an octave
** Aggregate complementation, the separation of pitch-clas ...
proteins
and
antibodies.
However, there are now known to be at least 50 proteins that act as opsonins for pathogens or other targets.
Mechanisms
Opsonins induce phagocytosis of targets by binding the targets (e.g. bacteria) and then also binding phagocytic receptors on phagocytes. Thus, opsonins act as bridging