Uptake, entrapment, and digestion of non-cellular particles in a cell specialized for this process is called phagocytosis. Particle entrapment occurs through the formation of cavities (phagosomes) that, after particle uptake, fuse with specialized vesicles called lysosomes. They contain the enzymes necessary for digestion or degradation of the trapped particles.
What is phagocytosis?
Phagocytosis is the intracellular digestion process of cells (phagocytes) specialized for this purpose. Phagocytosis is the term used to describe the intracellular digestion process of cells (phagocytes) specialized for this purpose. Phagocytosis involves the entire “feeding” process, which consists of the entrapment of the extracellular material to be digested and its degradation or digestion. Phagocytes flow around the material and enclose it in special cavities called phagosomes. Subsequently, small vesicles called lysosomes unite with the phagosome to form the phagolysosome and provide their digestive enzymes, which are used to initiate internal digestion. The material to be digested may be pathogenic bacteria, cells infected with viruses, metastatic cancer cells or cellular debris from dead cells, and fungi or fungal spores. Foreign substances and toxins that are in the bloodstream or have penetrated the tissue are also rendered harmless – if possible – via the pathway of phagocytosis. The particles that remain after phagocytosis are released by the phagocyte into the extracellular space, where the particles are usually taken up by the lymphatic system and released via a collection site into the venous blood for further disposal. Dendritic cells occupy a special position. They can also take up pathogenic germs and foreign or harmful substances, but they do not have the ability to phagocytose the ingested substances, but can only transport them further in the intercellular space.
Function and task
Phagocytosis is used by the innate and acquired immune system for the decentralized control of possible pathogenic germs, the decentralized degradation of endogenous dead cells, and the killing of endogenous cancer cells and cells infected with viruses. The phagocytic cells capable and specialized to perform these tasks are several types of granulocytes, white blood cells that belong to the innate or primary immune system and can spring into action without delay. Phagocytosis is thus called upon to perform several tasks and functions. One important task is to kill the pathogenic germs identified by the immune system and captured by the phagocytes, and then to break them down in such a way that the substances that can still be used are made available to the body and the “indigestible” and unusable remains can be excreted via the lymphatic system and the bloodstream. Another important task of phagocytosis is to break down the many dying or already dead endogenous cells, recover the reusable substances, and pass the unusable remainder to the lymphatic system for disposal. To perform these tasks, in addition to phagocytes, certain tissue cells such as fibroblasts, endothelial cells, and epithelial cells are also capable of limited phagocytosis, which is exclusively related to the removal of endogenous cellular debris. This relieves the phagocytes and makes them more available for their primary task, the defense against pathogenic germs. A third important task and function is performed by phagocytosis as a “vicarious agent” of the immune system in the killing and disintegration of the body’s own cells when these have been identified by the immune system as dangerous tumor cells or as cells infected by viruses. The recognition of the cells takes place via complex specific recognition patterns to which the phagocytes are programmed. The phagocytes’ responses are controlled by cytokines, messenger substances to which the phagocytes respond with specific behaviors or activity patterns.
Diseases and disorders
Problems can arise in connection with phagocytosis, one of the purposes of which is to kill pathogenic germs, because certain pathogens, such as Mycobacterium tuberculosis (tubercle bacillus), are able to induce the phagocyte to undergo phagocytosis so that they can enter the interior of the phagocyte via this process without being digested. Within the phagocyte, quasi in its protection, the germs can then multiply strongly.Other pathogenic germs that use phagocytosis for their own reproduction are the bacterial pathogens Salmonella enterica and Shigella flexneri. Both germs are ingested with spoiled food, and both pathogens synthesize specific protein mixtures that they inject into phagocytes, causing them to form membrane protrusions that the germs then actively ingest. These bacteria also survive the attempt at phagocytosis unharmed and instead proliferate under the protection of the phagocyte. A serious limitation in the ability to phagocytose can result from phagocyte defects. These are a reduction in the number of phagocytes, neutropenia, or a restriction in the functional capacity of the cells, e.g., due to a leukocyte adhesion defect (LAD). Both diseases can be primary, i.e. genetically determined, or acquired through infections or toxic substances – also as a side effect of certain drugs. A so-called septic granulomatosis is present when neutrophil granulocytes, which are part of the primary immune system, actively ingest pathogens, but the subsequent phagocytosis cannot kill the pathogens due to a genetically determined metabolic disorder. Acquired or congenital disorders of T lymphocytes are associated with various autoimmune diseases. In this case, the dysfunction of T lymphocytes may cause them to not always recognize the body’s own tissue or cells as the body’s own. They attack the cells of certain types of tissue and then phagocytose them. A well-known viral disease associated with dysfunction of T lymphocytes is AIDS. The disease is caused by the HI virus and leads to a constant decrease in T-helper cells, so that in the advanced stage the immune defense completely slackens.
