Antibodies are primarily effective in controlling organisms with a prolonged extracellular stage in their lifecycle, such as most common bacteria, some viruses, and some eukaryotic parasites. Their principle protective activities are as follows.
1. Neutralizing antibodies bind free extracellular particles, such as toxins and some virions, and prevent their actions on host cells.
2. Antibodies opsonize pathogens for phagocytosis. By opsonization, which literally means 'to spread with butter', antibody bound to a pathogen's surface can engage Fc receptors on the surface of phagocytes, activating their engulfment mechanisms.
3. IgM and IgG1, IgG2, and IgG3 isotypes can activate the 'classical' complement pathway. Complement is primarily an amplifying cascade of plasma proteinases and associated regulatory proteins. Binding of the first complement component C1q to an antibody-antigen complex activates the cascade and leads to the deposition of many molecules of C3b on the target antigen. C3b opsonizes for phagocytosis and activates the assembly of the lytic 'membrane attack complex', which then punches holes in the surface of the target cell.
Complement can also be activated without antibody by certain common bacterial and fungal cell-wall components ('alternative pathway'). The alternative complement pathway can also act to amplify the classical route. Complement by-products activate mast cells and act as chemoattractants for neutrophils, monocytes, etc.
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