Opsonins and Dysopsonins

Immediately after intravenous administration, the nano-particles encounter hundreds of different plasma proteins. A protein layer begins to form within seconds at the nano-particles' surface after contact with blood. The adsorption of plasma proteins that are capable to interact with receptors on the MPS cells, thus promoting the particle uptake by these cells, is called opsonization.

The complement is considered to be one of the main opsonins involved in the uptake of intravenously administered particles [13, 14]. It comprises about 20 blood proteins that can interact in a complex cascade involving specific binding and proteolytic activation steps [15]. Fibrinogen is also one of the most active plasma proteins involved in phagocytosis [16]. Immunoglobulin IgG and fibronectin also have a strong opsonic activity [17, 18].

Opposite the opsonin family, certain blood components called dysopsonins were shown to inhibit phagocytosis. Such effects mainly were reported in the case of polystyrene particles [19, 20] and liposomes [21]. It was suggested that the principal factors that result in a dysopsonic action are two serum components, one with a molecular weight below 30 KDa and another one with a molecular weight higher than 100 KDa [22].

It is believed that one key in controlling the particle uptake is the simultaneous reduction of opsonin adsorption and the selective adsorption of dysopsonins [23]. Special attention was given to the mechanism by which particles interact with proteins.

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