Immune System

The immune system protects the body from diseasecausing microorganisms. It consists of two levels of protection, the non-specific defenses and the specific defenses. The non-specific defenses, such as the skin and mucous membranes, prevent microorganisms from entering the body. The specific defenses are activated when microorganisms evade the non-specific defenses and invade the body.

The human body is constantly bombarded with microorganisms, many of which can cause disease. Some of these microorganisms are viruses, such as those that cause colds and influenza; other microorganisms are bacteria, such as those that cause pneumonia and food poisoning. Still other microorganisms are parasites or fungi. Usually, the immune system is so efficient that most of us are unaware of the battle that takes place almost everyday, as the immune system rids the body of harmful invaders. However, when the immune system is injured or destroyed, the consequences are severe. For instance, Acquired Immune Deficiency Syndrome (AIDS) is caused by a virus—Human Immunodeficiency Virus (HIV)—that attacks a key immune system cell, the helper T-cell lymphocyte. Without these cells, the immune system cannot function. People with AIDS cannot fight off the microorganisms that constantly bombard their bodies, and eventually succumb to infections that a healthy immune system would effortlessly neutralize.

For the immune system to work properly, two things must happen: First, the body must recognize that it has been invaded by foreign microorganisms. Second, the immune response must be quickly activated before many body tissue cells are destroyed by the invaders.

How the immune system recognizes foreign invaders

The cell membrane of every cell is studded with various proteins that protrude from the surface of the The antibody-mediated immune response. Illustration by Hans & Cassidy. Courtesy of Gale Group. membrane. These proteins are a kind of name tag called the Major Histocompatibility Complex (MHC). They identify all the cells of the body as belonging to the "self." An invading microorganism, such as a bacterium, does not have the "self" MHC on its surface. When an immune system cell encounters this "non-self" cell, it alerts the body that it has been invaded by a foreign cell. Every person has their own unique MHC. For this reason, organ transplants are often unsuccessful because the immune system interprets the transplanted organ as "foreign," since the transplanted organ cells have a "non-self" MHC. Organ recipients usually take immunosuppressant drugs to suppress the immune response, and every effort is made to transplant organs from close relatives, who have genetically similar MHCs.

In addition to a lack of the "self" MHC, cells that prompt an immune response have foreign molecules (called antigens) on their membrane surfaces. An antigen is usually a protein or polysaccharide complex on the outer layer of an invading microorganism. The antigen can be a viral coat, the cell wall of a bacterium, or the surface of other types of cells. Antigens are extremely important in the identification of foreign microorganisms. The specific immune response depends on the ability of the immune lymphocytes to identify the invader and create immune cells that specifically mark the invader for destruction.

How the two defenses work together

The immune system keeps out microorganisms with non-specific defenses. Non-specific defenses do not involve identification of the antigen of a microorganism; rather the non-specific defenses simply react to the presence of a "non-self" cell. Oftentimes, these non-specific defenses effectively destroy microorganisms. However, if they are not effective and the microorganisms manage to infect tissues, the specific defenses are activated. The specific defenses work by recognizing the specific antigen of a microorganism and mounting a response that targets the microorganism for destruction by components of the non-specific system. The major difference between the non-specific defenses and the specific defenses is that the former impart a general type of protection against all kinds of foreign invaders, while the specific defenses create protection that is tailored to match the particular antigen that has invaded the body.

The non-specific defenses consist of the outer barriers, the lymphocytes, and the various responses that are designed to protect the body against invasion by any foreign microorganism.

Barriers: skin and mucous membranes

The skin and mucous membranes act as effective barriers against harmful invaders. The surface of the skin is slightly acidic which makes it difficult for many microorganisms to survive. In addition, the enzyme lysozyme, which is present in sweat, tears, and saliva, kills many bacteria. Mucous membranes line many of the body's entrances, such as those that open into the respiratory, digestive, and uro-genital tract. Bacteria become trapped in the thick mucous layers and are thus prevented from entering the body. In the upper respiratory tract, the hairs that line the nose also trap bacteria. Any bacteria that are inhaled deeper into the respiratory tract are swept back out again by the cilia—tiny hairs—that line the trachea and bronchii. One reason why smokers are more susceptible to respiratory infections is that hot cigarette smoke disables the cilia, slowing the movement of mucus and bacteria out of the respiratory tract. Within days of quitting smoking, the cilia regenerate and new quitters then cough and bring up large amounts of mucus, which eventually subsides.

Non-specific immune cells

Non-specific lymphocytes carry out "search and destroy" missions within the body. If these cells encounter a foreign microorganism, they will either engulf the foreign invader or destroy the invader with enzymes. The following is a list of non-specific lymphocytes:

Macrophages are large lymphocytes which engulf foreign cells. Because macrophages ingest other cells, they are also called phagocytes (phagein, to eat + kytos, cell).

Neutrophils are cells that migrate to areas where bacteria have invaded, such as entrances created by cuts in the skin. Neutrophils phagocytize microorganisms and release microorganism-killing enzymes. Neutrophils die quickly; pus is an accumulation of dead neutrophils.

Natural killer cells kill body cells infected with viruses, by punching a hole in the cell membrane, causing the cell to lyse, or break apart.

The inflammatory response

The inflammatory response is an immune response confined to a small area. When a finger is cut, the area becomes red, swollen, and warm. These signs are evidence of the inflammatory response. Injured tissues send out signals to immune system cells, which quickly migrate to the injured area. These immune cells perform different functions: some engulf bacteria, others release bacteria-killing chemicals. Other immune cells release a substance called histamine, which causes blood vessels to become wider (dilate), thus increasing blood flow to the area. All of these activities promote healing in the injured tissue.

An inappropriate inflammatory response is the cause of allergic reactions. When a person is "allergic" to pollen, the body's immune system is reacting to pollen (a harmless substance) as if it were a bacterium and an immune response is prompted. When pollen is inhaled it stimulates an inflammatory response in the nasal cavity and sinuses. Histamine is released which dilates blood The cell-mediated immune response. Illustration by Hans & Cassidy. Courtesy of Gale Group. vessels, and also causes large amounts of mucous to be produced, leading to a "runny nose." In addition, histamine stimulates the release of tears and is responsible for the watery eyes and nasal congestion typical of allergies.

To combat these reactions, many people take drugs that deactivate histamine. These drugs, called antihistamines, are available over the counter and by prescription. Some allergic reactions, involve the production of large amounts of histamine which impairs breathing and necessitates prompt emergency care. People prone to these extreme allergic reactions must carry a special syringe with epinephrine (adrenalin), a drug that quickly counteracts this severe respiratory reaction.

Complement

The complement system is a group of more than 20 proteins that "complement" other immune responses. When activated, the complement proteins perform a variety of functions: they coat the outside of microorganisms, making them easier for immune cells to engulf; they stimulate the release of histamine in the inflammatory response; and they destroy virus-infected cells by puncturing the plasma membrane of the infected cell, causing the cell to burst open.

Specific immune cells

Two kinds of lymphocytes operate in the specific immune response: T lymphocytes and B lymphocytes, (T lymphocytes are made in the thymus gland, while B lymphocytes are made in bone marrow). The T and B lymphocytes migrate to other parts of the lymphatic system, such as the lymph nodes, Peyer's patches, and tonsils. Non-specific lymphocytes attack any foreign cell, while B and T lymphocytes are individually configured to attack a specific antigen. In other words, the blood and lymph of humans have T-cell lymphocytes that specifically target the chickenpox virus, T-cell lymphocytes that target the diphtheria virus, and so on. When T-cell lymphocytes specific for the chickenpox virus encounter a body cell infected with this virus, the T cell multiplies rapidly and destroying the invading virus.

Memory cells

After the invader has been neutralized, some T cells remain behind. These cells, called memory cells, impart immunity to future attacks by the virus. Once a person has had chickenpox, memory cells quickly stave off subsequent infections. This secondary immune response, involving memory cells, is much faster than the primary immune response.

Some diseases, such as smallpox, are so dangerous that it is better to artificially induce immunity rather than to wait for a person to create memory cells after an infection. Vaccination injects whole or parts of killed viruses or bacteria into the bloodstream, prompting memory cells to be made without a person developing the disease.

Helper T cells

Helper T cells are a subset of T-cell lymphocytes which play a significant role in both the cell-mediated and antibody immune responses. Helper T cells are present in large numbers in the blood and lymphatic system, lymph nodes, and Peyer's patches. When one of the body's macrophage cells ingests a foreign invader, it displays the antigen on its membrane surface. These antigen-displaying-macrophages, or APCs, are the immune system's distress signal. When a helper T cell encounters an APC, it immediately binds to the antigen on the macrophage. This binding unleashes several powerful chemicals called cytokines. Some cytokines, such as interleukin I, stimulate the growth and division of T cells. Other cytokines play a role in the fever response, another non-specific immune defense. Still another cytokine, called interleukin II, stimulates the division of cytotoxic T cells, key components of the cell-mediated response. The binding also "turns on" the antibody response. In effect, the helper T cells stand at the center of both the cell-mediated and antibody responses.

Any disease that destroys helper T cells destroys the immune system. HIV infects and kills helper T cells, so disabling the immune system and leaving the body helpless to stave off infection.

B cells and the antibody response

B-cell lymphocytes, or B cells, are the primary players in the antibody response. When an antigen-specific B cell is activated by the binding of an APC to a helper T cell, it begins to divide. These dividing B cells are called plasma cells. The plasma cells, in turn, secrete antibodies, proteins that attach to the antigen on bacteria or free viruses, marking them for destruction by macrophages or complement. After the infection has subsided, a few memory B cells persist that confer immunity.

An antibody does not itself destroy microorganisms. Instead, the antibody that has been made in response to a specific microorganism binds to the specific antigen on its surface. With the antibody molecule bound to its antigen, the microorganism is targeted by destructive immune cells like macrophages and NK cells. Antibodytagged microorganisms can also be destroyed by the complement system.

T-cell lymphocytes are the primary players in the cell-mediated response. When an antigen-specific helper T cell is activated by the binding of an APC, the cell multiplies. The cells produced from this division are called cytotoxic T cells. Cytotoxic T cells target and kill cells that have been infected with a specific microorganism. After the infection has subsided, a few memory T cells persist, so conferring immunity.

Chemical signals activate the immune response and other chemical signals must turn it off. When all the invading microorganisms have been neutralized, special T cells (called suppressor T cells) release cytokines that deactivate the cytotoxic T cells and the plasma cells.