Structural Biochemistry/Phagocytosis

Overview

Phagocytosis refers to the cellular ingestion and digestion of bacteria and other foreign substances. This word refers to "cellular eating" whereas pinocytosis refers to "cellular drinking". Phagocytosis enables macrophages and dendritic cells to present antigens to and stimulate helper T cells, which in turn stimulate the B-cells whose antibodies contribute to phagocytosis. Phagocytosis can also be witnessed through amoebas and other protists engulfing smaller organisms or food particles. Additionally in phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a food vacuole. The particle will be digested after the food vacuole fuses with a lysosome containing hydrolytic enzymes.

Phagocytosis is an effective nonspecific immune response. Phagocytosis recognizes alien cells and particles. For the phagocytosis to proceed, macrophages and neutrophils must first recognize the surface of a particle as foreign. When a phagocyte surface interacts with the surface of another body cell, the phagocyte becomes temporarily paralyzed. So, paralysis allows the phagocyte to evaluate whether the other cell is friend or foe, self or nonself. During phagocytosis, the cytoplasmic membrane of the phagocyte flows around, and then engulfs, the bacterium, producing an intracellular phagosome. Overall, phagocytosis is selective for particles recognized as foreign to the body. Oxygen-independent and oxygen-dependent mechanisms of killing are initiated by fusion between a lysosome and a bacteria-containing phagosome.

Primary phagocytosis (or known as phagoptosis) has recently recognized as a form of cell death caused by phagocytosis of viable cells, which causes cell destruction. Viable cells either loss their 'don't eat me' signals or exposing the 'eat me' signals, which causes their phagocytosis by phagocytes.

Beside destroying microorganism, phagocytes also release regulatory molecules that diffuse to other cells to help coordinating the overall response to infections.

Functions of Phagocytosis in immune system

The Immune System heavily relies on the phagocytosis process to fight infection. These specialized cells "eat" microorganisms to destroy them.

Neutrophils: These cells eat pathogens in acute inflammation cells. Very high amount of these cells in our bloodstream.

Macrophages: These cells are closely related to monocytes in the blood and long-lived cells which help in chronic inflammation cases.

Dendritic: Phagocytosis in these cells help relay messages for specific responses rather than actually fighting and destroying the pathogens.

B Lymphocytes: Phagocytosis in these cells help produce antibodies that help fight foreign invaders.

Mechanism of Phagocytosis

What Happens A pathogen is engulfed into a macrophage or neutrophil to become a phagosome. This is basically a container for the pathogen. Then lysosome is incorporated into the vessicle due to the destructive contents of lysosome. The outcome is a phagolysosome. The microbes inside the phagolysosome are destroyed by many factors. Some relevant factors are oxygen radicals, nitric oxide, anti-microbial peptides/proteins, binding proteins and hydrogen ion transport. These all contribute to the destruction of the microorganism.

The phagocytosis process can be sped up through the use of opsonins. These are molecules that bind to the surface of microorganisms to mark them for faster and more effective attacks. Some special bacteria present problems to the phagocytosis process. For example, the tuberculosis bacteria prevents the lysosome from binding to the phagosome, thus preventing destruction by phagocytosis. Other bacteria have a capsule around them which protects against phagocytosis.

Functions of regulatory molecules

Cytokines, regulatory molecules released from phagocytes, regulate an immune response. The majority of cytokines are small proteins and mainly released from white blood cells and their relatives such as macrophages. Cytokines act similar to pancrines, regulatory molecules that released by one cell and diffuse locally to neighbor cells. Cytokines (TNFɑ and IL1) released from macrophages are important because they help coordinate an immune response.

Further reading

• Slonczewski, Joan L. Microbiology: An Evolving Science. Second Edition.