Opsonin

IgM opsonizes indirectly by activating complement. This produces C3b, which is an opsonin. 

Compared with phagocytosis, pinocytosis appears to be a universal phenomenon in all cells, including phagocytes. Unlike phagocytosis, which is mediated by the serum opsonin, pinocytosis does not require any external stimulus. Pinocytosis is divided into two types fluid-phase pinocytosis and adsorptive pinocytosis (see Fig. 3B). Fluid-phase pinocytosis is a nonspecific, continuous process, and it is believed to be useful as a general process for transporting macromolecular constructs through epithelia, some endothelia, and into various blood cells. Adsorptive pinocytosis, in... 

Several liposome-based drugs have been approved for clinical application [64]. One of the clinically approved liposomes is Doxil, a PEGylated liposome containing doxorubicin (DOX), which is used for the treatment of a number of diseases [65]. As shown in this case, in the field of liposome drug development, PEG is widely used to protect the liposome from recognition by opsonins, thereby reducing liposome clearance. 

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.

The precise function of many acute-phase proteins is not known. C-reactive protein binds lipids, whilst a-macroglobulin and ceruloplasmin can scavenge some reactive oxygen metabolites. However, many acute-phase proteins are glycoproteins and can bind to bacterial surfaces hence, they may serve as non-specific opsonins for phagocytosis, and their synthesis is stimulated by IL-1 and IL-6. 

When neutrophils encounter bacteria, possibly coated with opsonin proteins of the complement system, the invaders are engulfed by the phagocytes and taken into the cells by endocytosis. A small part of the neutrophil membrane is used to create a phagosome - that is, a vacuole enclosing the bacterial cells. Within a matter of a few... 

After being marked by the adsorption of certain serum proteins, so ealled opsonines, they are identified as invaders and destroyed by specific immune eells, mainly in the liver, spleen, and bone marrow. 

Opsonization. Certain complement factors ( opsonins ) bind to the pathogens and thereby mark them as targets for phagocytos-ing cells (e.g., macrophages). 

As with normal hydrocarbon-based surfactants, polymeric micelles have a core-shell structure in aqueous systems (Jones and Leroux, 1999). The shell is responsible for micelle stabilization and interactions with plasma proteins and cell membranes. It usually consists of chains of hydrophilic nonbiodegradable, biocompatible polymers such as PEO. The biodistribution of the carrier is mainly dictated by the nature of the hydrophilic shell (Yokoyama, 1998). PEO forms a dense brush around the micelle core preventing interaction between the micelle and proteins, for example, opsonins, which promote rapid circulatory clearance by the mononuclear phagocyte system (MPS) (Papisov, 1995). Other polymers such as pdty(sopropylacrylamide) (PNIPA) (Cammas etal., 1997 Chung etal., 1999) and poly(alkylacrylicacid) (Chen etal., 1995 Kwon and Kataoka, 1995 Kohorietal., 1998) can impart additional temperature or pH-sensitivity to the micelles, and may eventually be used to confer bioadhesive properties (Inoue et al., 1998). 

---