Opsonins receptors

Figure 1.4. Recognition of bacteria by neutrophils. Invading bacteria are opsonised by serum proteins, such as complement fragments (e.g. C3b) and immunoglobulins. The plasma membranes of neutrophils possess receptors for these opsonins (e.g. Fc receptors and complement receptors). Thus, occupancy of these opsonin receptors triggers phagocytosis and activates events such as the respiratory burst and degranulation. Note that the receptors and opsonins are not drawn to scale.

Tosi ME, Zakem H, Berger M. Neutrophil elastase cleaves C3bi on opsonized pseudomonas as well as CRl on neutrophils to create a functionally important opsonin receptor mismatch. J Clin Invest 1990 86 300. 

Bacteria bind to complement components and the bacterium-complement complexes bind complement receptors on the surface of macrophages. Phagocytosis may also be mediated by specific antibodies that function as opsonins, which bind to particles, rendering them susceptible to phagocytosis. The bacterium-antibody complex then binds the macrophages via the Fc receptor and phagocytosis begins. 

Receptors for complement proteins that serve as leucocyte activators (e.g. C3a and C5a see section 2.2.4) or complement proteins that serve to coat (to opsonize) microorganisms (e.g. C3b). An opsonized microbial surface more readily adheres to a phagocyte membrane, with the opsonin triggering enhanced activity of the phagocyte itself. 

Activation of either pathway of complement results in the cleavage of C3 to C3b and C3a. The latter is released into the circulation and acts as an anaphylatoxin and a chemotaxin. C3b, with its activated thiol ester, binds to membranes and then activates the subsequent factors in the membrane-attack complex. C3b also acts as an opsonin, binding to receptors on phagocytic cells and resulting in the ingestion of bacteria, viruses, and other foreign particles by these cells. 

Fig. 1 The innate immunity receptor complexes in inflammation (SIRS/Sepsis) and cardiovascular disease. The basic cluster includes specific and nonspecific opsonin recognition sites (ex vivo, control) whereas Ugand-induced clustering is dependent on ligand surface composition (LPS/LTtA or ceramide/HSP70 and atherogenic LDL). (Adapted from Pfeiffer et al. 2001)...

Both oral and parenteral uptake of colloidal carrier systems have been found to depend on the nature of the carrier as such. In the latter case, the RES uptake of colloidal drug carriers depends on a number of factors, notably the surface properties of the carrier (see above). This is related to the adsorption of certain serum proteins (opsonins) at the carrier surface, which initiates various biological responses. For example, it is known that macrophages, major components in the RES system, have Fc receptors at their surfaces, which means that carriers with adsorbed IgG are more likely to be captured by these cells (52). By reducing the adsorption of the opsonins at the carrier surface, e.g. by surface treatment using PEO derivatives, a very low serum protein adsorption can be reached, thereby prolonging the bloodstream circulation time and obtaining a more uniform tissue distribution (see above). 

Human macrophages possess receptor sites for IgG 1 and IgG3, but apparently not for IgM (136). Molecules of these subclasses are therefore superior to IgM as opsonins. 

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