An opsonin is a type of molecule that helps to bind an antigen to an immune cell. It can help cells called phagocytes link up with antigens; the process of opsonization is when the antibody molecules are coated with the binding agent. Otherwise the negative charge of the cells would repel the molecule as well as the invading bacteria, for example. The immune system typically has to recognize an invading particle as foreign before creating antibodies. In a process called phagocytosis, antibodies that are Y-shaped attach to the bacteria, which the phagocytes can bind to before digesting the invader.
The arms of the Y-shape molecule connect to a foreign particle, and the bottom part is where an immune cell can link to via a receptor. When the two bind together, the phagocyte can engulf the particle and digest it with enzymes. An opsonin typically coats the negatively charge molecules on the surface of a cell. The opsonin molecules generally serve to cover the antigens until they reach the cellular membrane. When this occurs in one cell, receptors in other nearby phagocytes are usually activated as well.
Opsonin molecules operate throughout the immune system. Dendritic cells can collect antigens once they come in contact with a germ or foreign particle. They are then typically directed to a lymph node or the spleen. The antigens are then presented to immune cells called lymphocytes, and an immune response can start. Fighting an infection this way can take a long time, but cells called macrophages can immediately destroy pathogens, and start fighting an infection within minutes.
Macrophages can have four types of molecules that recognize invaders. These can be formulated without exposure to a germ, while other molecules that recognize microbes are found in the blood. Such molecular varieties can include C-reactive protein, which is formed in the liver, and mannose binding lectin. Both of these are types of opsonin and bind to microbes to enable immune cells to absorb infectious particles.
An opsonin can also be an antibody and this type is sometimes created if the immune system responds in a certain way. Opsonization is often limited if a bacterium is encased in a carbohydrate capsule. The casing can make it hard for the immune system to recognize the microbe, and antibodies usually have a hard time breaking through to the cell surface. Opsonin compounds work throughout the immune system, and also help cells called neutrophils and monocytes to use antigens.