The Immune System -- Briefly
The medical literature on the immune system is gargantuan. Any attempt to cover immunology thoroughly would fill thousands of pages and, of course, we can't do that here. Instead, this chapter will briefly look at some of the cells and functions of the immune system in order to become acquainted with the immune side of the immune-brain connection.
Many cells of the immune system are fully functional at birth and need no special training. These cells comprise the innate arm of the immune system. Other cells, called lymphocytes, require environmental training to become fully functional. Lymphocytes comprise the adaptive arm (i.e. it adapts to the environment) of the immune system. We will look at the innate arm first.
All organisms, whether they are worms, insects or humans, are born with innate immunity. Innate immunity is essentially a series of unlearned defenses displayed by certain immune cells at birth and continue throughout the life of the organism. The main cells involved in innate immunity are monocytes, macrophages and granulocytes. This branch of the immune system does not learn from experience, so vaccines and previous infections have no effect on the functioning of these cells. Innate immunity has four basic functions: phagocytosis, antigen presentation, inflammation and tissue repair.
Phagocytosis Phagocytosis means to eat cells [phago=eating; cyto=of cells]. Phagocytes, such as macrophages and granulocytes, defend the body by eating invading bacteria, fungi and viruses. The also eat damaged, dead and malignant cells, foreign proteins and dangerous chemicals. Phagocytes are such voracious scavengers of bacteria and other unwanted cells that they usually burst from eating too much. Pus at infection sites, for the most part, is the ruptured remains of these selfless defenders of the body.
Phagocytes are the first line of defense against invading microorganisms, damaged tissue, malignant cells and dangerous chemicals. They "sense" danger at the chemical-biological level and then eat the danger. Phagocytes are a fundamental defensive and sensory part of the immune system.
Antigen presentation An antigen is a foreign molecule, usually a small protein derived from the cell wall of a bacteria, virus or fungus. When a phagocyte devours an invading microorganism, it breaks up the microorganism's cell wall into small pieces. The small protein pieces of the cell wall are the antigens. Antigens are like chemical fingerprints which identify the invading microorganism.
The phagocyte displays the chemical fingerprint (i.e. the antigen) on its own cell surface. This is called presenting the antigen, an extremely vital process. The phagocyte is presenting the antigen to lymphocytes, so the lymphocytes can read and identify the antigen. Once the antigen is identified, the lymphocytes can begin attacking and destroying the invading microorganism.
Inflammation The powerful defensive strategy used by innate immune cells to wall off diseased tissue or invading microorganisms is called inflammation. It is an attempt (usually successful) to isolate and localize every infection, trauma and tumor. Without inflammation, a simple infection like a sore throat or an infected toe nail could very easily extend throughout the body. If an infection becomes systemic, then the immune system is faced with fighting mammoth battles everywhere. Without antibiotics, the immune system, faced with these mammoth battles, is often overwhelmed. The final result of overpowering the immune system is death.
Many things activate innate immune cells--microorganisms, damaged tissue, foreign chemicals and malignant cells. Once innate immune cells are activated, in addition to eating the danger, they begin releasing cytokines (and other chemicals) which cause blood, tissue fluids and a wide variety of immune cells to flow into the infected or damaged site. This results in inflammation, that is, red, warm and swollen tissue. Cytokines are essential for the life saving process of inflammation to occur.
In the public's mind, inflammation is viewed as a harmful, destructive process. Something to avoid and stop as quickly as possible. The pubic is wrong on this one, because without inflammation a simple infection or trauma would become a life threatening crisis throughout the body. Whenever you have inflamed tissue, it means your innate immune system is attempting to save your life. Essentially inflammation is a sign that the innate immune system is activated and working hard to defend the body.
Tissue Repair Another vital function of the innate immune system is tissue repair. One of the first steps in tissue repair is the cleaning up of injured tissue caused by trauma or infection. Phagocytes routinely clean up injured tissue by eating dead or damaged cells and tissue debris.
After the damaged tissue is devoured, innate immune cells coordinate the manufacture of new tissue to replace the injured tissue. Cytokines and other chemicals released by activated innate immune cells stimulate endothelial cells, fibroblasts and chondrocytes to make new tissue. By using cytokines and other chemicals, the innate immune system stimulates and coordinates the actions of many different cells needed for tissue repair. The ability to initiate and coordinate tissue repair is an extraordinary power of the innate immune system.
Cell Types Innate immune cells, for the most part, are made in bone marrow, released into the blood and then invade tissues throughout the body. It is very difficult to analyze the immune system in the invaded tissues. Instead, blood is used for analysis, because it is easy to access and measure. Unfortunately, blood is not a very good window to view the immune system, because most of the important immune actions are taking place in tissues other than blood. Blood merely transports immune cells from one tissue to another, whereas the great immune system battles against invading microorganisms, tumors and noxious chemicals usually take place in solid tissues.
Less than 1% of the cells in blood are immune cells, the other 99% are red blood cells. Immune cells are white so they are called white blood cells (wbc) or the equivalent term, leukocyte (leuko=white; cyte=cell). A high white count means that the blood is transporting more immune cells than normal from the bone marrow to a solid tissue in the body. A high white count is a reliable sign that the immune system is activated (i.e. engaged in a battle for survival) someplace in the body, but it doesn't say where the battle is taking place or many details about the battle.
Granulocytes In the cytoplasm of these cells are many granules, hence the name granulocyte. The three main types of granulocytes are: neutrophils, eosinophils and basophils. They are named according to the dyes used to stain them for microscopic analysis.
Granulocytes are made continuously in bone marrow, usually at modest rates. When mature, they are released into the blood and from there they migrate into solid tissues. If there is an infection in the body, say in the throat, then the phagocytes in the throat will be activated and release cytokines, several of which stimulate bone marrow to produce large numbers of granulocytes, thereby dramatically raising blood levels of granulocytes. Huge number of granulocytes will migrate from blood to the throat (attracted by cytokines) to help in the battle against the invading bacteria.
Granulocytes have very brief lives. They only live from 6 to 36 hours in blood and then another 2 to 5 days in tissues before they die. They are usually the first new immune cells to arrive at damaged or infected tissue. Hence, the first few days of any inflammatory process is dominated by granulocytes.
Neutrophils are stained by neutral dyes, thus the term neutrophil. Over 60% of the wbc's are neutrophils, so they are the most prevalent type of wbc. During the first few days of an infection their numbers can go up five fold. The tremendously elevated white count during the acute phase of an illness is primarily due to the rapid rise in neutrophil population.
The symptoms and signs during the first few days of an infection are pretty much due to neutrophil activity. They can be rapidly deployed in large numbers to any site of infection or tissue damage. These quick acting phagocytic cells secrete many enzymes, inflammatory substances and cytokines. When the immune system is calm (i.e. not activated), the vast majority of neutrophils reside in the blood and very few can be found in other tissues. If the immune system becomes activated and secretes cytokines, then neutrophils quickly leave the blood and invade the site of infection or tissue damage. In addition, during immune activation, huge numbers of new neutrophils are made in the bone marrow, migrate into the blood and then invade the infection site.
Eosinophils are stained by a dye called eosin, thus the name eosinophil. Only about 2% of the wbc's are eosinophils. These phagocytic, inflammatory cells play key roles in defenses against parasites and foreign proteins, but not against typical infections. Most eosinophils are present in the intestinal tract and the lungs, where parasites and foreign proteins are more likely to enter the body. Eosinophils secrete many enzymes, some inflammatory substances, but no cytokines.
Basophils are stained by basic dyes, hence the name basophil. Less than 1% of wbc's are basophils. They are particularly involved in allergies and inflammation, although they are not true phagocytes. Basophils secrete many inflammatory substances, but no cytokines.
Macrophages The macrophage is one of the most important cells of the immune system. For many years it has been known as the main phagocyte of the immune system. Macrophages eat and destroy pathogens, such as bacteria, viruses and fungi, thus they are a critical part of the body's defense against microorganisms. They also eat damaged, dying, and aberrant cells (such as malignant cells).
During the past 20 years scientists have discovered that the macrophage is much more than a phagocyte. The macrophage is one of the most complex and diverse chemical factories in the body. It has the ability to manufacture over 100 powerful chemicals ranging from cytokines and hormones to enzymes and prostaglandins. It can make many of the same powerful hormones secreted by the brain, pituitary and adrenal cortex. For example, beta-endorphin, a broad spectrum hormone which helps regulate the brain and the immune system, is secreted by both the brain and macrophages. Activated macrophages also produce adrenocorticotrophin hormone (ACTH). ACTH is an extremely potent pituitary hormone which stimulates the adrenal cortex to manufacture a wide variety of hormones necessary to combat stress, infection and trauma. Table I lists the macrophages' remarkable chemical arsenal, including cytokines.
The macrophage is a very ancient cell. It very likely evolved over a billion years ago when multicellular animals first appeared on earth. Multicellular organisms require phagocytic cells like the macrophage for survival. All animals, including jelly fish, star fish, worms, ants, spiders, clams, lobsters, dogs, monkeys and humans contain vast numbers of these remarkable cells.
Macrophages are produced in the bone marrow, where red blood cells, platelets and granulocytes are made. In the bone marrow, macrophage precursor cells are called promonocytes. Promonocytes eventually mature and migrate into the blood, where they are called monocytes. Monocytes are only found in the blood. After about 40 hours in the blood, monocytes begin moving into solid tissues. They invade every tissue and organ in the body, including brain, bone, liver, spleen, pancreas, gut, skin, lymph glands, muscles and arteries.
Once a monocyte invades a tissue, it matures further and is transformed into a macrophage. Macrophages are larger, more powerful than monocytes. They reside in their chosen tissue until they die, macrophages are never found in the blood. Thus, there are no blood tests to investigate, measure or evaluate macrophages. Consequently, one of the most powerful and complex cells in the body, namely the macrophage, is rarely evaluated in health and disease. This is one reason why very few people, except experts studying this cell, are aware of the importance of the macrophage in almost all diseases.
Macrophages live in every tissue (with the exception of blood), gland, organ, orifice, surface and lining of the body. They are found throughout the lining of the gastro-intestinal tract and uro-genital tract. They line the respiratory tract, including mouth, nose, throat and lungs. Every millimeter of skin contains macrophages. Brain, bone, muscle, kidney, spleen, thymus, pancreas, liver, adrenals, cerebrospinal fluid, thyroid, synovial fluid, joints, lymph glands, pituitary, prostate and ovaries are rich with macrophages.
Until recently, due to the difficulty in identifying macrophages under the microscope, they have been misnamed in most tissues. For example, osteoclasts (bone dissolving cells) in bone are in fact macrophages. Microglia in brain, histiocytes in skin and connective tissue, synoviocytes in joints, Kupffer cells in the liver, misangial cells in the kidney, foam cells in arteries and Langerhans cells in skin are all macrophages. The historical misnaming of the macrophage is another reason few people, outside of experts doing research, are aware of these cells' importance in almost every disease known to man.
During typical periods of good health, macrophages are resting, quiescent and secrete very little if any of their potent chemicals They are primarily defensive sentinels, alert and vigilant, patiently waiting for invading pathogens, malignant cells or trauma. Once macrophages detect danger, they become activated and begin secreting powerful cytokines such as interleukin-1, tumor necrosis factor and interferon-alpha and many other chemicals in its arsenal. The cytokines command the brain, liver, immune system, endocrine system and various other tissues to act as one unit in the urgent defense of the body. With their powerful, broad spectrum effects, the cytokines in a sense declare 'martial law' and take over command and control of the body.
These smart cells can distinguish between friend and foe. For example, the gastrointestinal tract is always loaded with bacteria, yet intestinal macrophages are normally calm and quiescent. But, if unfriendly bacteria or viruses enter the gut, then macrophages lining the intestinal tract quickly become activated and declare martial law by secreting cytokines. The severe, debilitating symptoms of food poisoning and 'stomach flu' are vivid examples of macrophage (and other immune cells) activation and their 'declaration of martial law'.
Microorganisms are not the only things that can activate macrophages. Damaged tissue, dying tissue, malignant cells, foreign tissue, various chemicals and many unknown factors can awaken macrophages. Chronic macrophage activation occurs in many diseases, such as, rheumatoid arthritis, lupus, multiple sclerosis, osteoporosis and Alzheimer's Disease. Much of the tissue destruction, inflammation and emotional pathology occurring in these dreaded diseases is mediated by chronically energized macrophages. The cause of the chronic macrophage activation in these and many other diseases is unknown.
The Double-Edged Sword The awesome power of the macrophage is a double-edged sword. Its lethal strength is necessary in order to defend the body against invading pathogens, noxious chemicals, trauma, dying tissue and malignant cells. This same lethal strength, unfortunately, can also injure the body. During long term, chronic macrophage activation severe damage to the body can occur. Rheumatoid arthritis and multiple sclerosis are prime examples of this. The tissue destruction in these diseases for the most part is caused by chronically activated macrophages. Chronic diseases where the immune system is causing most if not all of the pathology, are called auto-immune diseases. Rheumatoid arthritis, multiple sclerosis and lupus are well known examples of auto-immune disease. Chronic macrophage activation is a central part of most, if not all, auto-immune diseases.
Monocytes Since monocytes are the precursors of macrophages, monocytes can be thought of as immature macrophages residing in the blood. Monocytes make the same cytokines as mature tissue macrophages, but they lack the ability to synthesize many of the other chemicals made by the macrophage. Monocytes do have phagocytic ability. This is very important since pathogens can enter the blood via transfusions, dirty needles, trauma or by breaching inflammatory barriers at infection sites. Monocytes, for example, are the main cells eating the HIV virus found in the blood of AIDS patients. Unfortunately monocytes are unable to destroy the HIV virus.
In ordinary clinical settings solid tissues from patients are rarely sampled and evaluated, except for biopsies to screen for malignant cells. The only tissue routinely evaluated in patients is blood, so monocytes can be routinely evaluated. Blood monocytes do give some hints as to what tissue macrophages may be doing. For example, a finding of increased numbers and activities of monocytes in blood suggests that there are increased numbers of activated macrophages someplace in the body. Of course a high monocyte count doesn't reveal the location of the macrophage activation or the reason for the activation.
Adaptive immunity is only exhibited by vertebrates. Cells of the adaptive branch do acquire new abilities and powers (i.e. they adapt) as the body grows, matures and interacts with a biologically hostile world. In contrast, innate immune cells facing a microbe infested world.do not develop new abilities as the body matures,
Lymphocytes The principal adaptive immune cells are called lymphocytes, since they were first discovered concentrated in lymph. Lymph (lympha=water) is the yellowish watery tissue fluid that collects and flows in special passageways called lymphatic vessels. Most of the lymphatic vessels empty into blood vessels below the neck. Lymphocytes circulate throughout the body, migrating from lymph to blood, then from blood to tissues and finally from tissues back to lymph.
At various strategic sites in the lymphatic system are structures called lymph nodes. These important lymph glands are lined with macrophages and lymphocytes. They function as immunological filters and purifiers of lymph. Microorganisms or foreign proteins which have invaded tissue spaces will eventually flow into a lymph node. There they will be eaten and processed by macrophages.
During gestation, immature lymphocytes are made in the bone arrow. One main type of lymphocyte, called a B-lymphocyte, is matured in the bone marrow. The other main type of lymphocyte, called a T-lymphocyte, is matured in the thymus. By early infancy all of the millions of varieties of B and T-lymphocytes have matured. The mature T-lymphocytes migrate from the thymus to locations throughout the body, including spleen, lymph nodes, tonsils, lymph and various pockets of lymphoid tissue elsewhere in the body, especially in intestines. The mature B-lymphocytes migrate from bone marrow in a similar fashion.
After early infancy, new lymphocytes are made in lymph nodes and other lymphoid structures in the body. Once an individual is born, bone marrow is no longer an important site for lymphocyte formation.
About 20% of the wbc's in blood are lymphocytes. Only 2% of the body's lymphocytes are in the blood at any one time, hence blood samples only give a very incomplete picture of lymphocyte activity throughout the body. The remaining 98% of the lymphocytes reside throughout the body in lymph, lymph nodes, tonsils and various organs like liver, spleen and intestine.
Lymphocyte Activation The ability to recognize and selectively attack specific, unique antigens is the powerful and defining feature of lymphocytes. In any one person there are millions of different and unique lymphocytes. Each unique lymphocyte can recognize one unique antigen. Thus, in each individual, the millions of different lymphocytes taken together can recognize millions of unique antigens.
In order to be activated, lymphocytes must be exposed to an antigen processed by a macrophage or other antigen presenting cell. A number of steps are involved in activating lymphocytes. Some of the essential steps are:
- Macrophages ingest, process and display the antigen of the invading microorganism (presenting the antigen). Lymphocytes only respond to antigens being presented by macrophages or certain other antigen presenting cells. This often takes place in lymph nodes or other lymph glands.
- The activated, antigen presenting macrophages begin secreting cytokines such as interleukin-1. IL-1 alerts and attracts other immune cells, including lymphocytes, to the invasion site or lymphoid organ. Lymphocytes cannot be activated without the presence of IL-1.
- Millions of T-lymphocytes stream past the antigen being presented by the macrophage. The T-lymphocytes are checking to see if their antigen receptors can recognize the antigen being displayed by the macrophage. Eventually there will be a T-lymphocyte with an antigen receptor that recognizes and adheres to the displayed antigen.
- After recognizing the unique antigen, the unique T-lymphocyte is powerfully activated, resulting in the rapid production of an identical clone of T-lymphocytes which can recognize the unique antigen. Many of the T-lymphocytes in the clone can directly attack and kill any cell containing the identified antigen. These extraordinarily lethal cells are called cytotoxic T-lymphocytes or killer T-lymphocytes.
- The potent cytokine interleukin-2 is secreted by the activated T-lymphocytes. The clone of unique T-lymphocytes cannot be produced without IL-2. In addition, IL-2 tells B-lymphocytes to check the antigen being displayed. Eventually a unique B-lymphocyte will recognize and adhere to the displayed antigen. After adhering to the antigen, the unique B-lymphocyte is powerfully activated, resulting in the rapid production of an identical clone of B-lymphocytes. The identical clone of B-lymphocytes begin secreting a unique antibody which clumps and helps destroy any cell containing the identified antigen.
Adaptive (Acquired) Immunity The first time an individual's lymphocytes are exposed to a specific antigen, the response is relatively slow and weak. The second time the individual is invaded by the same kind of microorganism the lymphocyte response is more rapid and massive. The lymphocytes have learned from their previous experience and have acquired a more rapid, efficient response. This exemplifies adaptive immunity, that is, the lymphocytes adapt to a previous exposure of a microorganism by responding more rapidly and effectively to the next exposure of the same microorganism. This is the basis for vaccines being able to enhance immunity to specific infective agents. Adaptive immunity is the reason an initial viral infection in an individual, say chicken pox, provides that individual with lifelong immunity to the chicken pox virus.
Our description of the events occurring during a microbiological invasion have been greatly abbreviated. The immune system response to any foreign invader, whether it be a chemical, parasitic worm, fungus, bacteria or virus, is bewildering in its complexity. For instance, we only mentioned a few cytokines and a few of their actions, but there are actually 41 different potent broad spectrum cytokines produced by immune cells during the life and death battles with invaders. Also other immune cells, like natural killer cells were not mentioned, nor were the great variety of lymphocyte subsets. Nevertheless our brief coverage will provide us with some background for understanding the profound immune system involvement in psychiatric disease.
When activated, macrophages can secrete up to thirteen different cytokines, making them the greatest cytokine factory in the body. They are:
- interleukin-1 (IL-1) macrophage-colony stimulating factor (MCSF)
- interleukin-6 (IL-6) granulocyte-colony stimulating factor (GCSF)
- interferon-alpha (INFa) macrophage derived growth factor (MDGF)
- epidermal growth factor (EGF) transforming growth factor-beta (TGFb)
- tumor necrosis factor-alpha (TNFa) platelet derived growth factor (PDGF)
- basic fibroblast growth factor (FGF) heparin-binding growth factor (HBGF)
- granulocyte macrophage-colony stimulating factor (GMCSF)
All of these cytokines have remarkable abilities to regulate immune cells. The macrophage, via its cytokine secretions, may be the supreme director of the immune system. Table II lists the macrophage cytokines along with some of their actions. In the next section just one of the cytokines (interleukin-1) will be briefly discussed in order to give the reader a feeling for the extraordinarily diverse regulatory power of cytokines.
A number of the macrophage cytokines act as growth factors. Growth factors stimulate various cell types to grow and proliferate resulting in tissue growth and repair. Consequently, macrophages not only clean up the debris around damaged, injured or infected tissues, but they also are the main instruments for repairing damaged, injured or infected tissue.
Interleukin-1 This remarkable cytokine secreted by activated monocytes and macrophages regulates and controls a great variety of cells, tissues and organs in addition to immune cells. Interleukin-1 (IL-1), for example, has potent effects on the liver. It commands the liver to make five different proteins, called acute phase proteins. At the same time IL-1 instructs the liver to stop making two other important proteins named albumin and transferrin. IL-1 can also cause muscles, connective tissue and bones to waste away. In contrast, IL-1 stimulates other cells to make more bone and connective tissue. Endothelium, kidney, adipose tissue, pancreas, synovial fluid and all types of inflammatory cells can be controlled by IL-1. The extremely broad biological effects of IL-1 are unmatched by any classical hormone, but there are other cytokines, such as IL-2, IL-6 and TNF that have similar broad biological effects.
IL-1 has powerful effects on the brain. Receptors for IL-1 are found in many areas of the brain including five critical brain structures - the hypothalamus, the pituitary, the hippocampus, the raphe nucleus and the locus coeruleus. They can be profoundly regulated by IL-1. These brain centers control in a fundamental way most of the body's hormones and many of the brain's neurotransmitters, including norepinephrine, dopamine and serotonin. Instinctive behavior, emotions and basic drives such as hunger, thirst, sex, pleasure and pain are mediated by these key brain structures as are the reactions to physical and emotional stress. Few people, except for scientists specializing in cytokines, are aware of IL-1's profound ability to regulate brain activity.
IL-1 is also a necessary and fundamental activator of the immune system. It can activate and regulate all the cells of the innate immune system. The lymphocyte arm of the immune system requires IL-1 for activation. IL-1's control of lymphocytes illustrates thee macrophage's ability to regulate lymphocytes.
Next chapter: A Few Cytokines and Their Actions