Opsonisationrefers to the process whereby an opsonin binds to a particular amino acidsequence on an antigen which is then recognised by an immune cell and in turn, facilitatesa phagocytic response in the immune cell, often macrophages or neutrophils.
Antibodiescontain a variable region (which binds to the antigen) and a constant regionwhich is what the immune cell will recognise. In this assay the opsonins usedwere immunoglobulin G (IgG) as well as C3b which was found in guinea pigcomplement. IgG contains a variable region which recognised and bound to anamino acid sequence on S. aureus, this sequence is known as the epitope. IgGhas 2 variable regions (antigen binding sites) which it used to coat thebacteria while its constant region known as the Fc region is recognised byneutrophil Fc receptors. Using its Fc receptors, the neutrophil binds to theopsonin bringing the neutrophil in close proximity with the bacterial cellleading to phagocytosis of the coated bacterial cell. C3b is another opsoninwhich binds antigen and is recognised by complement receptor 1 on the immunecell which binds to C3b, causing the immune cell to initiate phagocytosisThe granules in neutrophils contain various AMPs and digestiveenzymes which are used to kill phagocytosed bacteria.
There are primary,secondary and tertiary granules which undergo “degranulation” to release theircomponents and attack the bacteria. An example from primary granules(Azurophils) are defensins which are AMPs that rupture bacterial membranescausing efflux of bacterial cellular components. An example from secondary(specific) granules is lactoferrin which is a protein that binds Iron,specifically from the bacteria which is essential for its survival and growth.An example from a tertiary granule is lysoszyme, an enzyme that binds andhydrolyses components of the bacteria’s peptidoglycan cell wall.