The Immune System

The immune system is a complicated network of special cells and organs that make up a vital part in our bodily systems. The immune system plays a very important role as its primary responsibility is to defend the body against the attack by 'foreign' invaders such as: bacteria, viruses and fungi. These invaders attack the cells in the body and try to reproduce causing viruses and diseases. The immune system has three different lines of defense which it uses to challenge the invaders. The body is a beautiful, delicate temple which can be described as a castle and the attack from the invaders could be considered as an attack on the castle. The immune system can be considered to be the army that protects the castle. Our body, the castle, is constantly under threat from these invaders, or antigens, and our army, the immune system, works extremely hard to offer to the castle continuous protection.

The first line of defense is where the immune system creates a barrier such as, the skin (castle wall) and mucous membranes (moat) that prevents the invaders, known as antigens, from initially entering the castle. The skin provides a nearly impassable barrier that secrets lactic acid and lysozymes. Together these two substances slow down bacterial growth by breaking down their cell walls.

When the outside barrier is intact, it is nearly impossible for the invaders to enter the castle however, the invaders occasionally find openings that they can pass through, for instance, the nose, eyes, mouth, open wounds, etc. These openings are also protected from invasion by lysozyme. The nasal cavity, or nose, is lined with mucus that has the ability to trap foreign invaders, and thus preventing them from entering the respiratory tract, the tunnel. Any invaders that manage to enter into the tunnel are suddenly stopped by the mucus and the cilia, the soldiers, before being marched back out of the castle the same way they entered. Any invaders that are lucky enough to sneak past the soldiers eventually enter the stomach, which is filled with an acid that destroys the invaders.

If the first line of defense is unsuccessful at preventing the invasion, the invaders are then attacked by the second line of defense. The second line of defense is where your immune system will try to detect the virus and abolish it before it begins to spread and can be described as the guards who stand on the walls protecting the castle. This is known as cell-mediated immunity. It is here that the invaders are that are under attack are captured and then marched along the lymphatic system, to the blood stream, where the white blood cells, or corporals, are in charge of dealing with the invaders. This process is known as phagocytosis and this is where the "white cell engulfs the pathogen, takes it into a vacuole inside the cytoplasm and then digests it with lytic enzymes." (Boyle et al, 2008, P.314). The regiments of white blood cells that are responsible for this are known as the phagocytes. Phagocytes are made of neurophiles, the most common type of white blood cell, and monocytes. These soldiers, or white blood cells, only survive for several days after having destroyed the invaders.

The third line of defense attacks specific pathogens and will only come into effect if the virus manages to reproduce and starts to cause problems. This is more commonly known as inflammatory response and is where the lymphocytes, specialist soldiers, who are capable of handling specific immune responses come into play. Lymphocytes are produced by the stem cells located in the bone marrow, their army base camp, and react when they are instructed to defend the body. There are two types of lymphocytes, the T-cells and B-cells.

The T-cells are processed by the thymus gland, there are only a small amount of T-cells located in thymus as the body limits the quantity to protect itself from damage and it is here that they mature and prepare for the attack. The B-cells are produced and processed in the bone marrow before travelling via the lymph to the lymph nodes, which is like a waiting room, where they wait until there services are required. There main function is to seek and destroy antigens.

The B-cell is a lymphocyte that develops into an antibody-secreting cell when it comes into contact with the appropriate antigen for which it was coded. A B-cell does not fight the invader directly instead it is equipped with weapons, called antibodies. The antibodies then travel in the blood to the site of infection. B-cells have specific receptors on their surfaces which will only bind to specific antigens. Once a B-cell has encountered this antigen, it is stimulated to divide rapidly to produce millions of identical cells which eliminate the virus. There are five different types of antibodies which are produced by the B-cells; IgA, IgD, IgE, IgG and IgM. All of these antibodies are immunoglobin's, they have a wide range and variety of functions within the immune system and are all capable of identifying hundreds of thousands of invaders and producing the appropriate antibodies.

The antibody IgA is found in the body secretions, such as breast milk and saliva, and prevents antigens from crossing the epithelial membranes and invading deeper tissues. The antibody IgD is displayed on the surface of the B-cells, this is where the antigen bind to activate the B-cells. The antibody IgE is found on the membranes of certain cells, such as basophils and mast cells, and if it binds to its antigen the inflammatory response is activated. This antibody is more commonly found in excess in an allergy. The antibody IgG is the largest, most common antibody type and attacks many different pathogens. It is the only antibody that is capable of crossing the placenta during pregnancy, to protect the fetus. And finally, the antibody IgM is produced in large quantities and is distributed throughout the lymph and blood.

IgM is the first type of antibody that is called to respond to the infection, this is known as the primary response. The structure of the IgM antibody is different to the other antibody and this allows its response to be so effective.

When the specific antibody identifies the invader it reacts by suddenly multiplying, forming a clone, which secretes a large amount of the specific antibodies required. Although the response may be sufficient to deal with the antigen, the majority of the antibodies are no longer required and will expire, unless there is another encounter with the same antigen within a short period of time.

The antibody IgG is responsible for initiating the secondary response. The secondary response is a quick process in which some of the B-cells that survive after attacking the infection and are given a new role within the army as memory B-cells. This is an important role as some of the invaders will attempt to re-enter the castle. To avoid another invasion these memory cells are given the instruction to defend the castle. "These memory cells 'remember' what the pathogen is like and, if it tries to invade again, they all divide rapidly to produce an even greater number of active B cells." (Boyle et al, 2008, P.317). B-cells do not have the capability of carrying out responses on their own and in this case, the T-cells are required to help.

T-cells are a type of lymphocyte which, unlike the B-cell, do not secrete antibodies. They are very specific in their role, as they are responsible for identifying the cells in the body that contain viruses and once these cells have been identified, they are responsible for eliminating them.

There are different types of T-cell; killer T-cells, helper T-cells and suppressor T-cells. T-cells are instructed to help defend the body when the antigens begin penetrating the walls of the body's cells.

Helper T-cells are responsible for helping, or rather controlling the immune response. They stimulate and give instructions to the B-cells to multiply and produce antibodies, they also activate the other types of T-cells and instruct the macrophages to prepare for the process phagocytosis.

Killer T-cells are like the soldier standing on the front line. When the killer T-cells receives the instructions from the helper T-cells, they start to attack. They attacke by puncturing holes in the cell wall of the invader, which causes the infected cell to lose cytoplasm and ultimately results in the death of the invader.

Suppressor T-cells act like the negotiator between the cells. They are responsible for suppressing or reducing the immune response to an antigen. This action prevents unnecessary damage to the body and an unnecessary waste of resources.

In conclusion, it can be said that the immune systems defense mechanisms are well designed to allow sufficient protection for the body. If the first line of defense is broken, then the body is still protected by the second and third line of defense. If all three lines of defense fail then the body is then open to attack and may result in a serious infection or disease.

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