Mechanisms of the Immune System: An Overview

When the human body is attacked by microbes or pathogens, it defends itself using certain mechanisms. There are two types of mechanisms – one which is non-specific and the other which is specific to the attack. The non-specific mechanism is similar no matter what attacks the body and is again sub divided into external and internal defences.

The first line of defences

The body’s first line of defense against pathogens is mostly physical. It involves sweat, skin, tears, mucus and stomach acid. Our skin and mucous membraneswhich line the body passages, are fairly effective in keeping most pathogens out of the body. They act like a protective barrier, defending against viral and bacterial invaders. The skin cannot be penetrated by bacteria or viruses under normal conditions. It has a pH range of 3-5 which is acidic enough to prevent the growth of bacteria. The clotting of blood near open wounds prevents an open space for antigens to easily enter the organism by coagulating the blood, and Lysozymes are an enzyme found in tears and saliva that have powerful digestive capabilities, and can break down foreign agents to a harmless status before they enter the body. Mucus in the nose traps pathogens, which are then washed away or destroyed by chemicals.

The respiratory tract also has its own line of defense. Invading microbes and debris are trapped in layers of mucous or they are filtered by tiny hair like structures called cilia. The cilia move in waves, sweeping the debris towards the entrance where they can be extracted through coughing or sneezing, so don’t hold your sneezes in!AndCorrosive acids in the stomach and protein digesting enzymes destroy most of the invading organisms carried in by food.

The first line makes a very powerful line of defence, but sometimes intruders can find their way past this wall. Thankfully we have a second line on the inside!

The second line of defence

The second line of defence is immobilized when invaders enter in the body. A nonspecific internal defence mechanism is the process of phagocytosis; the ingestion of invading bacteria by certain blood cells. There are many phagocytic responses used in the body. When foreign particles penetrate the skin; like in cases of injury, certain leukocytes known as monocytes move to the blood tissues where they develop into eaters called macrophages. Using pseudopods, the macrophages engulf and destroy the bacteria with their enzymes.

Another phagocytic response: white blood cells. Neutrophils are attracted to chemical signals sent off by cells that have been damaged by microbes. In the process of called chemotaxis, neutrophils move towards infected tissue. The neutrophils then engulf the microbe and release lysosomal enzymes that digest both the microbe and the leukocyte. The remaining fragments of protein, dead white blood and digested remainder are called pus. Tissue damage which is caused by physical damage also initiates an inflammatory response. An inflammatory response is a non-specific immune response which results in swelling, redness, heat and pain. Pus and inflammation are signs that the second line of defence is at work.

That is how your body responds to a small localized injury. The body can also respond to severe injuries with a system-wide defence. Injured cells emit chemicals that simulate the production of phagocytic white blood cells and increase their release into the bloodstream.

Bone marrow, the source of all blood cells is found in the inner spongy part of the upper leg bone, upper arm bone, breastbone and shoulder blades.

A fever is a good example of the body’s system-wide response to infections. When infectious organisms spread through the body like in cold or flu, neutrophils and macrophages digest the invaders and release chemicals into the bloodstream. These chemicals cause your body to reset its thermostat to a higher temperature such as 40 degrees Celsius. These conditions in the body make it difficult for the harmful bacteria to survive; so essentially, the fever helps you recover. People often take medication like aspirin to reduce fever; this however may actually prolong the infection. If the temperature is above 40 degrees Celsius however, it can be unsafe. Keep in mind human cells themselves cannot survive over 43 degrees Celsius.

THE IMMUNE SYSTEM

The immune system is made up of cells, protein, tissues and organs. It defends us from germs and bacteria every day. The immune system is usually very successful but sometimes when there is a problem we can get infections and illnesses.

About the Immune System

The immune system is the body’s defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms that invade body systems and cause disease.

The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. The cells involved are white blood cells called leukocytes. Leukocytes are large opaque blood cells that engulf invading microbes and produce antibodies.They combine to seek out and destroy disease-causing organisms or substances. All leukocytes are produced in bone marrow. There are approximately 6,000,000,000 leukocytes in the human body. They are rather colorless because they don’t contain hemoglobin which makes red bloodcells red. They have a life expectancy of 2-3 days and therefore the body is constanlty making large amounts of leukocytes all the time. Picture this: there is half a million white blood cells per very drop of blood!

Leukocytes are produced or stored in many locations in the body, including the thymus, spleen, and bone marrow. This is why they are called the lymphoid organs. Leukocytes are also located in clumps of lymphoid tissue all around the body. They have asymetrical shapes which can change enabling them to get around all obstacles.

The leukocytes circulate all around the body between the organs and nodes through the lymphatic vessels and the blood vessels. By doing so, the immune system works in a coordinated way to constantly monitor the body for germs or substances that might cause problems.

There are 2 basic types of Leukocytes: They are phagocytes and lymphocytes. Phagocytes are cells that chew up invading organisms and lymphocytes are cells that allow the body to remember and recognize previous invaders and help the body destroy them.

There are different types of phagocytes. The most common type is the neutrophil, it primarily fights bacteria. They are the most common type of white blood cell, neutrophils make up 50-70% of white blood cells in the body. They are the first to arrive at infected areasand kill intruders by ingestion. This process is called chemotaxis. Once the pathogen is swallowed the neutrophil dies.

The process of neutrophils killing bacteria involves them releasing a net of fibers which traps the cell. This is called neutrophil extracellular trap (N.E.T). Some people have neutrophil difficiencies and as you can imagine it leads to severe problems and a compromised immune system.

Other types of phagocytes are besophils, Eosinophils, Monocytes and Macrophages. Besophils are very rare in the body, they make up less than 1% of white blood cells. They d not do much as a result and simply help respond to infections. Eosinophils help ingulf parasites and discharge destructive enzymes to damage invading cells. They also kill parasitic eggs and worms. Monocytes are also very rare in the body, the less monocytes in the body the better. They replenish Macrophages and also help against infection. Finally we have Macrophages. They are cells that eat invaders and are involved in primary and innate immun response. For example they can be found in the lungs where they clean foreign debirs so they do not cause any problems. They usually remain stationed at specific posts in the body where foreign materials often enter. Anything that passes by the macrophages is scanned and if something is suspicious they engluf it. Once englufed the macrophage cereates an enzyme that neutralizes the invader so it becomes harmless and connot replicate and they preserve the antigen so that in the future it can be recognized and dealt with faster.

The two kinds of lymphocytes are B lymphocytes and T lymphocytes. Lymphocytes start out in the bone marrow and from there they either stay and mature into B cells or they leave for the thymus gland, where they mature into T cells. B lymphocytes and T lymphocytes have different functions. B lymphocytes are like the body’s military intelligence system, they seek out their targets and send defenses to lock onto them and track them down. T cells on the other hand are like the soldiers, destroying the invaders that the intelligence systems (B cells) have identified.

Antibodies are very specific Y-shaped proteins. For example an antibody produced against the influenza virus is not effective against HIV. These Y-shaped tails of the proteins are very similar, no matter which type of anti-body. Variation may only be produced in the outer edge of each arm, the area where the anti-body combines with the antigen. Antigen markers are different depending on the virus, for example the antigen marker of the influenza virus is different from that of the HIV virus. Each antigen is accompanied by its own antibody, shape wise. The markers of an antigen are located on the membrane of the virus or bacterium. After each attachment of an antibody, the antibody-antigen combination makes it more conspicuous, making it easy to be destroyed by wandering microphage.

Antibodies prevent poisons and toxins from destroying cells with receptor sites, found on different cells. This might explain why some poisons affect the nervous system, while others affect digestive or circulatory systems. This receptor site is designed to hold either a hormone or a specific nutrient. Specialized geometrical structures allow toxins and poisons to become attached to the receptor sites on cell membranes. After bring attached, the poison is ingested by the cell, which the receptor site mistakes for hormones or nutrients, absorbing the poison. Antibodies then interfere with the poison so that the structure created is not recognized by the receptor cell. Receptor sites are also used by viruses as entry ports. As the virus injects its hereditary material into the cell, it leaves an outer protein coat in the entry port. The outer coat allows the virus to rest in different locations. For example the cold virus has the geometrical shape to allow it to attach the lung cells.

How the Body Recognizes Harmful Antigens

As the T-cell scouts, it looks for foreign bodies posing a threat to its survival. The macrophages then attack the invader, engulfing it. As the macrophage presses the antigen into its cell membrane, it couples with the T-cell, also known as a helper T-cell. The T-cell then reads the shape of the antigen and releases a chemical messenger called lymphokine. The B-cells divide into clone’s because of the lymphokine. A second message is then sent from the helper T-cell to the B-cell, allowing the production of antibodies. A specific type of antibody is produced by each B-cell. Antibodies are attached to their cell membranes by the time the B-cells enter the circulatory system.

The Killer T-cell is an activated additional defender if the helper T-cell. These lymphokines go out for one purpose, which is to destroy. After being activated, the killer T-cells puncture through the cell membrane of the invader, which may differ from a protozoan parasite to a bacterium. Viruses are very different from the rest, as they hide within the structure of the cell. As the viral coat is found attached to the cell`s membrane, the T-cell attacks the infected cell. Killing the infected cell prevents the reproduction of the virus.

Mutated cells are also destroyed by Killer T-cells. This process is extremely important as some of the altered cells may be cancerous. Getting cancerous virus or not depends on the success of the Killer T-cell. The body’s` rejection of organ transplants depend on Killer T-cells. Antigen markers on the organ will be recognized as foreign, sending the Killer T-cells in. Immunosuppressant drugs such as cyclosporine can slow down the Killer T cells. After the battle is done, and won, a different type of T-cell, the suppressor T-cell, signals the immune system to shut down. Communication between Suppressor T-cells and Helper T-cells is vital, allowing the immune system to know how many antibodies to produce to contain the antigens. Phagocytes clean the area, removing the dead and injured cells, and tissue begins to repair and replace.

As previously mentioned, helper T-cells must read a blue print of the invader before the B-cells can produce antibodies. This information is stored so that later infections can be destroyed before the microbe can cause any harm. Immunity is based on maintaining a good number of antibodies.

It’s believed that memory-B cells are generated during the infection to hold an imprint of the antigen or antigens that characterize the invader. Most b and t cells produced die off within a few days; but memory B-cells remain. It can identify the enemy and quickly mobilize antibody-producing B-cells to defeat invading pathogens before they can establish themselves. As long as the memory B-cell survives, a person is immune to the disease, which is why a person cannot catch chicken pox more than once.

Proteins also play a role in the body’s line of defence. When foreign organisms are present in the body, antimicrobial plasma proteins called complement proteins are activated. There are about 20 known types of complement proteins. Under normal conditions these proteins are inactive. When invading microbes trigger them however they in turn work as messengers. The complement proteins gather and initiate an attack on the cell membranes of the foreign organism. The proteins will then form a protective coating around the invader; this seals the invading cell immobilizing it. A second group punctures the cell membrane, this causes water to rush in and burst the cell. The tiny microbes become less soluble and more susceptible to phagocytes by leukocytes.

All of these specialized cells are parts of the immune response system that offer the body protection against disease. This protection is called immunity.

The Several Types of Immunity

Innate Immunity

Innate immunity is often called natural immunity, everyone is born with it, it is a general protection. It refers to basic resistance to germs or diseases that other species possess. For example, if a human has HIV their dog won’t get HIV because it has innate immunity to that disease. Innate immunity works in both ways something that makes us sick won’t get another species sick and something that gets another species sick won’t get us sick. The Responses in innate immunity are Broad and non specific. The molecules used have a limited repertoire of recognition. It is a first line of defense.

Innate immunity includes the external barriers of the body, like the skin and mucous membranes, which are the first line of defense in preventing diseases from entering the body. If this outer defensive wall is broken, the skin attempts to heal quickly and the second line of defence becomes involved.

Adaptive Immunity

The second kind of protection is adaptive/active immunity, it develops throughout our lives. Adaptive immunity involves the lymphocytes and develops as people are exposed to diseases or immunized against diseases through vaccination. With all kinds of diseases constantly exposed we adapt out bodies by taking vaccinations to become immune to them.

Passive Immunity

Passive immunity isan immunity borrowed externally from another source and it is temporary. For example, antibodies in a mother’s breast milk provide the baby with temporary immunity to diseases. This helps protect the baby against infections during childhood when the baby’s body is still in early stages of development and not very strong.

Everyone’s immune system is different. Some people never seem to get infections,while others seem to be sick all the time. As we grow older, we gradually become more immune to more germs as our immune system comes into contact with more and more of them.

Matching tissues for organ transplant

The main challenge is the immune response of the recipient- their immune systems ability to distinguish between self and non-self. The donor organ is often identified as an invader by distinct markers on its cell membrane, MHC or Major histocompatibility complex. MHC is a protein fingerprint unique to each person so the recipient will make antibodies to destroy what it thinks is a foreign invader.

To reduce rejections, attempts are made to match donor MHC with that of the recipient as closely as possible. For living donor transplants, physicians usually look to close relatives because the MHC is genetically controlled. The better the match, the greater the chances of long term success.

To help reduce rejection factor in deceased donor transplants, (because deceased donor transplants cannot usually have as close of an MHC as relatives) immunosuppressant drugs can be given, but it will also reduce the immune system’s ability to fight off invading viruses and bacteria; placing these patients at risk of infections.

Malfunctions of the immune system:

Abnormal functions of the immune system can give rise to two types of problems: immune deficiency diseases and inappropriate attacks of the immune system against nonthreatening agents. Immune deficiency diseases may be caused by a foreign agent, such as the HIV virus which attacks t-cells, or a hereditary condition such as the severe combined immune deficiency SCID. Inappropriate or exaggerated immune response can also create problems. Hypersensitivity or response is where the immune system attacks normal cells in one’s own body, which can destroy tissues and organs.

• Allergies

Allergies occur when the immune system mistakes harmless cells for harmful invaders. If you’re allergic to peanuts, your body recognizes one of its proteins as dangerous. Although it’s safe, your body immobilizes the antibody strike force against it. Increased tissue swelling, mucous secretion, and sometimes constricted air passages are all part of the immune response. Dust, ragweed and strawberries, do not pose any threat to life but the immune response can be so severe that it becomes life threatening. A sever food allergy is called an anaphylactic reaction which involves the respiratory and circulatory systems accompanied by hives, itching and swelling. When you eat a food to which you’re allergic, the cells that believe they are in danger release histamine. It changes the cells of the capillaries, increasing permeability. The enlarged capillary causes the area to redden. White blood cells leave the capillary in search for the invader altering osmotic pressure. Proteins in extracellular fluids create another osmotic force to oppose it. Anaphylactic shock can occur very quickly and weakness, sweating and difficulty breathing indicate the condition. Medial precautions include carrying a kit with adrenaline (Epipen).

• Autoimmune Disease

The immune system can make mistakes and launch an attack on the body’s own cells. Mutated T and B cells are capable of attacking the body but are usually held in check be suppressor T cells. The failure of suppressor T cells can cause diseases such as arthritis and rheumatic fever, all of which can scar the heart muscle. Drugs and serious infections can weaken suppressor T cells leaving the body vulnerable to autoimmune disease. The number of suppressor T cells also declines with age, and this explains the commonness of arthritis in the elderly.

Stem cell research:

The answer for replacing damaged tissues may lie in stem cell research rather than transplantation. Stem cells can differentiate and develop into a variety of different tissues such as epithelial tissue, muscle tissue or nerve tissue. Stem cells in the skin continuously replace cells that are sloughed off, give rise to a wide range of blood cells that protect against foreign invaders and identify human cells that have mutated, such as cancer cells. Stem cells are pluripotent cells that can give rise to different types of body cells.