The term macrophage has been derived from two Greek words macro denoting "large" and phage meaning "to eat". When a macrophage comes across an alien substance, it has a tendency to encircle and destroy it. These special cells get going as what is known as a monocyte in the bloodstream and eventually grow into a proper macrophage when the body identifies anything that is wrong. In fact, the human body is truly very smart. Subsequently, the monocytes normally start circulating in the bloodstream. When the body identifies any infection, the monocytes move to the affected part of the body and start transforming into macrophages. In fact, monocytes are capable of transforming themselves into various different types of macrophages, subject to what is required by the body. Once they are created, the macrophages usually survive for several months. In addition, our body remembers the infection so that it is able to recall as well as react faster in case the same infection happens again.
It is important to note that macrophages have the ability to distinguish self from non-self, which makes them work in such a way that they do not cause any harm or cause damage to the normal and healthy cells or alter their functioning.
Basically, macrophages are a form of white blood cells (leukocytes) that consume or destroy alien substances in the body. These cells are an important part of our innate or primary immune response to several microbes and substances that invade our immune system. In addition, macrophages comprise a vital part of our body's acquired immune system. These cells are always constantly, but quietly working in different parts of our body eliminating bacteria, viruses as well as alien waste material even before they can create a problem and harm our body.
Similar to any other blood cell, macrophages too begin their journey from the bone marrow. Actually, the life cycle of macrophages begin with a type of cell known as monocyte. These cells are capable of developing into a macrophage when they are induced to do so. A number of monocytes move out to particular body areas - for instance the liver, where these they develop into dedicated and specialized macrophages and remain there. At the same time, other monocytes become free-floating macrophages.
In a way, these cells serve as security guards for our immune system. Some of them permanently remain at their regular place of work - close to areas from where alien substances generally get into our body. They consistently screen all the materials that pass through them and keep searching material that may prove to be harmful for our body. There are other macrophages that are not stationed at any particular body part, but usually keep roaming - keep patrolling and looking for any intruder that might have given a slip to the other macrophages stationed at particular points.
When macrophages come across something that it believes might be harmful for the body, they immediately engulf the substance and produce enzymes to deactivate the substance, thereby stopping it from reproducing inside the body. This process that involves engulfing the alien harmful substance and eliminating it is known as phagocytosis, which literally means "eating cells". Macrophages utilize phagocytosis to bring together antigens that they present to the helper T-cells. They alert the T-cells about an alien invader in the body and, in turn, the T-cells set off an immune response to eliminate the foreign substance from the body.
However, the complete mechanism of macrophages is yet to be understood fully, as scientist are still learning about the functioning of these exceptional cells and how to utilize them more effectively to keep us disease free. For instance, in the beginning researchers were of the view that these cells caused harm to their victim's DNA to stop them from replicating. However, findings of studies that were published in 2009 revealed that these cells generate specific enzymes that work in a different manner. Further study is needed on this subject to get detailed information regarding the manner in which our body responds to any infection and the various ways in which even the immune cells can malfunction causing harm to one's self.
It has been observed that sometimes macrophages, which are considered to be scavenger cells, can also be responsible for some various problems in our body. In fact, these cells have been found to be associated in enlargement of lesions like granulomas, which are due to chronic inflammation. In addition, these cells also have a vital part in the inflammatory process and when they turn out to be overactive, they can even cause harm, instead of defending the body from harmful alien substances. It has been found that macrophages may at times even aggravate specific forms of cancer. In fact, the HIV virus may also hijack macrophages and subsequently utilize to spread the disease in the body.
Macrophages have their origin in white blood cells known as monocytes. In fact, monocytes are the largest variety of white blood cells or leukocytes. The nucleus of a macrophage is large and usually kidney-shaped. Macrophages have a solitary nucleus. Macrophages form in the bone marrow and within one to three days of their formation, they start circulating in our blood stream. These cells leave the blood vessels and pass through the endothelium of the blood vessel to go into the body tissues. After arriving at their destination, the monocytes transform into macrophages. Alternatively, they may also transform into a type of immune cells known as dendritic cells. The dendritic cells help in developing our antigen immunity.
Macrophages developed from monocytes and are able to distinguish self and non-self are specific to organs or tissues which form their host. In case there are requirement additional macrophages in any particular tissue, the macrophages inhabiting the tissue produce a type of proteins that are known as cytokines. The cytokines help the responding macrophages to transform into the particular form of macrophage that is required. For instance, macrophages that are combating a particular infection generate cytokine that help the existing macrophages to become specialized in combating particular pathogens. Macrophages proficient in healing wounds as well as repairing damaged tissues grow from cytokines that are generated in response to the injured tissue.
Almost all tissues and organs in our body have macrophages and, outside immunity, they perform several other functions too. These cells help in sex hormone production in male as well as female gonads. Macrophages also help in developing the network of blood vessels inside the ovary. In fact, this is crucial for producing the vital hormone progesterone. This hormone has a vital role in transplanting the embryo in the uterus. Macrophages are also present in the eye and they help in developing the network of blood vessels in the eye, which is essential for good vision.
There are various examples of macrophages residing in different locations of our body. Some of these and their benefits in those locations are discussed briefly below.
Central Nervous System (CNS): Glial cells called microglia are present in our nervous tissue. These cells are exceptionally small and their work is to patrol the brain as well as the spinal cord and remove all cellular waste. In this way, they protect the central nervous system from harms caused by micro organisms.
Adipose Tissue: Macrophages are also present in adipose tissue and here their work is to protect the adipose tissue from any potential onslaught of microbes. In addition, they also assist the adipose cells to uphold the sensitivity of our body to insulin.
Integumentary System: Macrophages are present in the integumentary system as Langerhans cells. These cells are present in the skin and work as a part of the immune system and also assist in development of new and healthy skin cells.
Kidneys: In the kidneys, macrophages assist in filtering the microbes from the blood. At the same time, they also help in the development of ducts.
Spleen: Macrophages are present in the spleen’s red pulp and their function is to aid in filtering the damaged red blood cells (erythrocytes) and harmful micro organisms from the blood.
Lymphatic System: In the lymphatic system, macrophages are stored in the medulla (the central area) of the lymph nodes and they work to filter the lymph of harmful microbes.
Reproductive System: In the male and female reproductive system, macrophages are present in gonads and they help in the development of sex cells, steroid hormone production and also in the development of embryo.
Digestive System: Macrophages are also present in the intestines, where they keep an eye on the environment, thereby protecting the digestive system from various microbes.
Lungs: Macrophages that are present in our lungs are called alveolar macrophages. Their primary work is to get rid of microbes and various harmful particles that enter the respiratory surfaces.
Bone: Monocytes that develop into macrophages start their journey from the bone marrow. Hence, it is natural that macrophages would be present in the bones. In the bones, macrophages may transform into bone cells known as osteoclasts, which assist in breaking down bone and also reabsorb as well as assimilate the components of bone. Cells that develop into macrophages are formed as well as reside in the bone marrow's non-vascular sections.
While the main role of macrophages is protecting us from pathogens like bacteria and viruses, at times these microbes also dodge the immune system and infect our immune cells. Some examples where the macrophages are infected by microbes to cause diseases are HIV, adenoviruses and the bacteria that are responsible for tuberculosis. Aside from these diseases, macrophages have also been associated with development of some other ailments like diabetes, heart disease and even cancer. In fact, macrophages present in our heart facilitate in developing heart disease like atherosclerosis. The arterial walls of an individual suffering from atherosclerosis become fat owing deposit of plaques along the walls of the artery and chronic inflammation caused by the white blood cells (leukocytes).
When macrophages are present in fat tissues, they can be responsible for inflammation, which triggers adipose cells to become insulin resistant quite easily. In turn, this can, result in the persons suffering from diabetes. Macrophages are also responsible for chronic inflammation and this can facilitate development as well as growth of carcinogenic cells.
