The human immune system is made up of many different structures and processes. Organs such as the bone marrow, thymus, spleen and lymph nodes are involved in this function, essential for the production of immune cells.
In this article we will describe microglia-related functions and diseases one of these cells.
Microglia are a type of glial cell found in the central nervous system. The term is used to talk about a set of cells that perform similar functions, mainly related to immune defense and phagocytosis of potentially harmful elements for neurons.
The term “microglia” was coined in 1920 by Pío del Río Hortega, a disciple of the neuroscience pioneer Santiago Ramón y Cajal. The immune functions of these cells have been known since the time of their discovery, although knowledge about their characteristics has advanced in recent decades.
It is a very versatile type of glia: The structure of microglia varies depending on the functions performed by each cell, the place where it is located and the chemical signals it receives from neighboring neurons. We speak of “phenotype” to refer to the specific form that each microglia adopts.
They originate from progenitor cells of the same lineage as those that make up blood, probably located in the bone marrow or in the yolk sac attached to the embryo. Some of these cells migrate to the brain during intrauterine development; Once they have reached this structure, they differentiate as microglia.
Glial cells or glia are located in the nervous system, that is, in the brain, in the spinal cord and in the cranial and spinal nerves. They support neurons in different ways: they give them physical support, they nourish them and eliminate pathogens, damaged tissues and waste products, they promote the transmission of neuronal impulses through the formation of myelin…
Among the cell types that are classified as glia, the following stand out: astrocytes, which are essential for the structure and function of the blood-brain barrier, oligodendrocytes, which create the myelin sheaths of the central nervous system, and Schwann cells, which do so in the peripheral.
Functions of these cells
Microglia are mainly known for their immune and hygienic roles ; However, it also fulfills other varied functions, such as maintaining the balance of the extracellular environment of the nervous system or repairing damaged tissues.
1. Phagocytosis (waste removal)
These cells phagocytose (“devour”) different types of compounds from the central nervous system: injured and dead cells, debris, viruses, bacteria, neurofibrillary tangles neuritic plaques… After phagocytation, both the microglia and their target remain inactive, thus reducing the risk of altering the functioning of the nervous system.
2. Maintenance of homeostasis
Microglia send signals through cytokines to other cell types, such as neurons, astrocytes and T lymphocytes, also involved in the immune system. Among the consequences of this function, the regulation of the homeostasis of the extracellular environment stands out, as well as the promotion of inflammation.
3. Inflammation and damage repair
When tissue of the central nervous system is damaged or infected, microglia facilitate inflammation; In this way, the repair process of injured cells begins, throughout which these cells are very important.
Furthermore, if damage occurs to the spinal cord, microglia eliminate the affected neuronal branches, allowing new nerve connections to be created.
4. Antigen presentation
When a tissue becomes inflamed, T lymphocytes cross the blood-brain barrier and enter the central nervous system. Once here they join with microglia cells that have phagocytosed antigens (particles from which antibodies are produced); This enhances threat elimination and injury recovery.
5. Cell destruction (cytotoxicity)
Microglia have the ability to destroy bacteria, viruses, infected neurons and other cell types by releasing hydrogen peroxide and nitric oxide. Sometimes this response is excessively aggressive and damages significant amounts of healthy tissues, causing even greater brain damage.
Diseases related to microglia
Dysfunctions in microglia are associated with very diverse alterations. These cells seem to be significantly involved in neurodegenerative diseases such as Alzheimer’s in which neuritic plaques and neurofibrillary tangles accumulate in the brain: the cytotoxicity of microglia attacks healthy neurons adjacent to the damaged tissue.
Microglia cells play a similar role in the development of dementia due to infection with HIV, the AIDS virus. In fact, this disease also directly affects microglia, infecting them and promoting neurotoxicity. Microglia are also involved in other infectious diseases, such as herpes encephalitis and bacterial meningitis.
Research reveals that glia It is important in the appearance of neuropathic pain, which manifests itself in alterations such as allodynia or phantom limb syndrome. This is because they are activated in response to nerve damage and promote the chronic release of chemical compounds associated with the sensation of pain.
