Macrophages: what are they, characteristics and functions?

Let's see what macrophages are and how they participate in biological processes to protect the organism.

The immune system makes life possible for anatomically complex living beings on the planet.. All animals are open systems, as we require the input of energy in the form of organic matter, oxygen in the air and a myriad of nutrients to carry out our metabolic functions.

For all these reasons, the digestive and respiratory systems are exposed to the outside world, as they communicate directly with the environment through the nostrils and mouth, respectively.

These routes of entry of gases and matter are a free runway for thousands of viruses, bacteria, protozoa and helminths, which settle in the mucous membranes of the tracts and sometimes travel through the circulatory system and invade the interior of the cells. To prevent the certain death of the host in the face of a pathogenic infestation, the immune system acts with various biological barriers.

From the skin to the lymphocytes, there are a number of mechanisms that prevent the proliferation of microorganisms in our body, in one way or another. The primary barriers (skin, mucous membranes, saliva, sweat and cough) prevent the entry of pathogens, but when these are established for any reason, a series of cellular bodies belonging to the "innate immune system" come into action, working in a general and rapid way against invasions. Macrophages are one of the most important cellular bodies in this process, and in this process, and here we tell you all about them.

What are macrophages?

Macrophages are cell bodies of an immune nature derived from Blood monocytes, which in turn are produced in the bone marrow by specialized precursors.which in turn are produced in the bone marrow by specialized precursors. It is noteworthy that, unlike other circulating elements, these cells have a rather marked polymorphism, being spherical, ovoid and polygonal in shape, with about 12-15 microns in total diameter. They also have an ovoid and nearly empty nucleus, with very little chromatin.

Originating from monocytes, macrophages leave the circulation to settle and differentiate into various tissues. These cell bodies are found in virtually all areas of the body and their main function is to phagocytose. Their main function is to phagocytose and digest any biological agent that is interpreted as dangerous or foreign to the organism.. In addition, it should be noted that there are several types depending on their location:

• Alveolar macrophage: as its name suggests, it is located in the pulmonary alveoli. Together, these cell bodies are responsible for the defense of the lower airways.
• Kupffer cells: responsible for immune response and tissue modulation in the liver.
• Microglia: located in the central nervous system. Eliminates old or dead neurons and exerts control over immunity in the nervous system.
• Spleen macrophages: responsible for eliminating old or dysfunctional red blood cells. This is extremely important, as it allows blood renewal.

Functions

As you can see, macrophages kill pathogens, but also the body's own cells that are no longer useful or are too old.. Below, we explore some of the general functionalities of macrophages and their most striking features. Don't miss it.

1. Phagocytosis

As their name suggests (macro, large and phage, to eat, "big eater"), the main function of macrophages is to phagocytize all foreign bodies that enter the organism, such as bacteria and metabolic waste substances.

Neutrophils (the most abundant white blood cells in the circulatory system, representing 70% of the total) are the first to arrive at the source of infection, but their lifespan is very short, from a few hours to a couple of days at most. The presence of neutrophils as the first internal defense barrier stimulates the macrophages, which will go to the site of action and begin by phagocytizing the dead neutrophils themselves.

Once they have encountered the pathogen, the macrophage envelops it with a phagosome or endocytic vesicle, a vacuole formed around a particle assimilated by phagocytosis, which then fuses with a lysosome internalized in the phagocytic cell, giving rise to a "phagolysosome". Finally, various enzymes and toxic peroxides act on the microorganism enclosed in the phagolysosome and destroy it.

2. Activation of the immune system

The macrophage destroys the localized threat in the specific tissue, but this is not the end of its function. Metabolic debris from the digestion of the microorganism is exposed on specific parts of the cell membrane of the phagocyte, allowing the threat to be identified by the phagocyte. This allows the identification of the threat by the T helper lymphocytes..

We are not going to focus on complex immune cascades, as it is sufficient to know that the recognition and multiplication of lymphocytes promotes the circulation of immunoglobulins (or antibodies). These antibodies recognize antigens on the surface of living microorganisms and adhere to them, acting as clear threat markers. Thus, the other macrophages know who to attack in a targeted manner and other cell bodies are set in motion to stop the infectious event.

The immune system is supposed to be divided into two strands: "innate" and "acquired". Macrophages are part of the innate mechanisms, but as you can see, the acquired immune system could not exist without them.. This leads us to two very clear conclusions:

• The innate immune system (especially macrophages and neutrophils) activate the acquired immune system in response to an infection.
• The acquired immune system uses the mechanisms of innate immunity to act. This means that, once antibodies have been produced and lymphocytes stimulated, macrophages continue to attack the threat.

3. Tissue repair

Beyond the ingestion of foreign compounds, macrophages also play an essential role in wound healing and tissue regeneration.

When an injury occurs, platelets are among the first circulating structures to reach the open area, as their function is to induce coagulation as quickly as possible in order to maintain internal homeostasis.

Attracted by the platelet growth factorsThe monocytes come through the bloodstream to the wound, with the help of vessel walls and fine capillaries.

They mature rapidly into macrophages, whose function is to to keep at bay any bacteria or pathogens that try to enter through the bleeding opening into the medium.. They are also responsible for getting rid of dead tissue (by releasing proteases), which "makes room" for rapid and effective tissue regeneration.

Macrophages and cancer: a double-edged sword

As it could not be otherwise, we must dedicate a few last lines to the role of these immune cells in the development of malignant neoplasms, since cancer is one of the most serious health problems worldwide today.

Unfortunately, the information is discordant in this area, and much less positive than might be expected. According to studies such as "Macrophage diversity enhances tumor progression and metastasis", published in the journal Cell, macrophages can promote and accelerate the growth and metastasis of cancer cells, contrary to what might be expected.contrary to what might be expected. The inflammatory events they provoke in early stages promote cell mutation to malignant strains, but they also stimulate the migration and proliferation of already cancerous cells, even inhibiting anti-tumor mechanisms.

Going even further, a high number of macrophages in a tumor correlates positively with a worse prognosis in many types of cancer, such as breast, cervical, brain or prostate cancer. Although not all the mechanisms that promote this problematic symbiosis are yet understood, this reality certainly puts into perspective that, sometimes, the human body kills itself without the help of exogenous factors..

Abstract

As we have seen, macrophages are essential cells for both innate and acquired immune responses, as well as for repairing tissues and maintaining hemostasis, among many other things. Furthermore, their study and characterization could help us to better understand why malignant tumors arise, as there seems to be a clear relationship between macrophages and cancer cells.

The future of cancer therapy is promising, as the current intention is to use these macrophages as true "Trojan horses". Because of their relationship with tumor cells, they could serve as reservoirs for anticancer drugs, even as a target for the development of targeted antitumor therapies.