Phagocytosis / Intracellular pathogen killing / Opsonization / Macrophage
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Phagocytosis
Phagocytosis is a process by which, a cell engulfs and eliminates large particles. In our body, macrophages are capable of ingesting and digesting exogenous antigens, such as whole microorganisms, foreign antigens as well as injured or dead host cells. In the first step in phagocytosis, macrophages are attracted towards these substances under the influence of various chemical substances generated by the immune response. This process is called chemotaxis. The next step in phagocytosis is the adherence of the antigen to the macrophage cell membrane. Adherence induces membrane protrusions, called pseudopodia, to extend around the attached material. Fusion of the pseudopodia encloses the material within a membrane-bounded structure called a phagosome. The phagosome moves toward the cell interior, where it fuses with a lysosome to form a phagolysosome. Lysosomes contain lysozyme and a variety of other hydrolytic enzymes that digest the ingested material. The digested contents of the phagolysosome are then eliminated in a process called exocytosis. Most of the products resulting from the digestion of ingested material are exocytosed, but some peptide products may interact with class II MHC molecules, forming complexes that move to the cell surface, where they are presented to TH cells.
The macrophage membrane has receptors for certain classes of antibody. If an antigen (e.g., a bacterium) is coated with the appropriate antibody, the complex of antigen and antibody binds to antibody receptors on the macrophage membrane more readily than antigen alone and phagocytosis is enhanced. Thus, antibody functions as an opsonin, a molecule that binds to both antigen and macrophage and enhances phagocytosis. The process by which particulate antigens are rendered more susceptible to phagocytosis is called opsonization.