Pathogens and disease

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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 pathogens such as: bacteria, viruses and fungi. These pathogens attack the cells in the body and try to reproduce causing viruses and diseases. The immune system has three different lines of defence which it uses to protect the body and to challenge the invaders. The body is a beautiful, delicate temple which can be described as a castle and the attack from the pathogens (invaders) could be considered as an attack on the castle. Our body, the castle, is constantly under threat from these invaders and our army, the immune system, works extremely hard to offer to the castle continuous protection.

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

When the castle wall is intact, it is nearly impossible for the invaders to enter the castle however, the invaders do occasionally manage to find openings that they can pass through for instance the nostrils, the eyes, the mouth, cuts and grazes, etc. These openings are also protected from invasion by the enzyme lysozyme. The nasal cavity, or nose, is lined with hairs and mucus that have the ability to trap the pathogens, thus preventing them from entering the respiratory tract. Any invaders that manage to enter into the respiratory tract are suddenly stopped by the mucus and the cilia, the soldiers, before being transported back out of the castle. Any invaders lucky enough to sneak past the soldiers continue down the respiratory tract where they then enter the stomach, which is filled with an acid that subsequently slows down the growth and ultimately destroys them.

If the first line of defence is unsuccessful, the invaders are then attacked by the second line of defence. The second line of defence is where your immune system will try to detect the virus and eliminate it before it begins to spread. This process is known as cell-mediated immunity. It is here that the attacking invaders are captured and then marched along the lymphatic system where the white blood cells, or corporals, are in charge of dealing with them. This process is known as phagocytosis and this is where the white blood cell surrounds the microbe, moving it into a vacuole inside the cytoplasm of the cell and then slowly destroying it with lytic enzymes. 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.

The third line of defence attacks specific pathogens and will only be initiated if the viruses manage to reproduce and problems start to occur. 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 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 amount in order 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. Their 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, which are then transported through the blood stream 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 (Waugh, Grant, 2006).

IgM is the first type of antibody that is called in, by the body, to respond to the infection, this is known as the primary response. The structure of the IgM antibody is different to the other antibody, as it is shaped like a star and exists as a pentamer. It's this shape that this allows the response from the IgM antibody to be so effective.

When the appropriate antibody identifies the invader it reacts by suddenly multiplying, forming a clone, which secretes a large amount of the appropriate 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. This is a quick process by which some of the B-cells that survive after attacking the infection and are given a new role as memory B-cells. This is an important role as some of the invaders will attempt to re-enter the castle. The main function of these memory cells is to 'remember' what the pathogen is like and they will spread throughout the body rapidly if the pathogen tries to re-enter the body. This is done to avoid another invasion on the body and these memory cells are instructed to defend the body. 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 kind of lymphocyte which, unlike the B-cell, does 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 system's defence mechanisms are intrinsically designed in order to allow sufficient protection for the body. If the first line of defence is broken, then the body is still protected by the second and third line of defence. If all three lines of defence fail, the body is then open to attack and may result in a serious infection or disease.


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Immunisation can be described as an artificial method of producing immunity.

Passive immunisation is where an individual is given an injection of antibodies that are made by another organism. This is useful in circumstances where the body would not have enough time to produce antibodies of its own. For example, if you are bitten by a snake, you may be given an injection of ready-made anti-venom, which is an antibody.

Active immunisation is where the body is stimulated to produce its own antibodies. This is achieved when a vaccine stimulates the immune system to produce the antibodies that fight against the disease. Active immunisation also helps teach the immune system how to produce the appropriate antibodies. After immunisation, if the individual comes into contact with the disease, their immune system will recognise it and will instantly begin to produce the antibodies required to fight it.

A newborn baby has a natural immunity to several diseases, such as measles, mumps and rubella, from antibodies that are passed from the mother to baby via the placenta and also via lactation of the mother's breast milk. Natural immunity only lasts a few weeks or months after birth and as such, the natural immunisation against diphtheria, tetanus, whooping cough and polio only lasts for about 2 - 3 months and in the case of measles, mumps and rubella it may last for up to one year after birth - this is why the MMR (measles, mumps and rubella) vaccine is given to children just after their first birthday. The timescales that are given for children's vaccinations are intended to increase protection in their bodies as their natural immunity wears off.


Influenza is a "highly infectious illness caused by a flu virus" (NHS Choices, 2010). The virus infects your airways and your lungs, causing a sudden fever, headache and general aches and pains. You may also lose your appetite, feel nauseous and havea dry, chesty cough. When an infected person coughs or sneezes, their droplet contains pathogens which will transmit the disease if breathed in by an uninfected person. It can also be spreadwhen someone, infected with the virus, touches everyday items andsurfaces, such as door handles or hand rails,with unwashed hands. The spread of the virus usually occurs in small outbreaks or in very rare cases, in epidemics. These outbreaks tend to occur more during the winter and generally spread very quickly, especially in places like schools and old age homes.

There are three different types of the influenza virus, which are called, A, B and C. The influenza A virus infects mammals and birds, whereas the Influenza B and C virus only affects humans. The A type of the influenza virus is the type most likely to cause epidemics and pandemics, due to the fact that the virus can undergo antigenic drift, this is when two strains of the virus combine to form a new strain with a combination of the antigens on its surface. The virus can also mutate naturally in an attempt to resist the body's defences such as the human immune system. Populations tend to have more resistance to influenza B and C because they cannot change by antigenic drift and can only change by natural mutation; therefore influenza B and C are usually similar to its predecessor and similar enough to be destroyed by the immune system.

An individual who has been attacked by virus C, requires antibodies (proteins made by the immune system), which provide immunity against the virus. The type C virus is only a mild infection which is not much different to the common cold and none of the symptoms mentioned above apply to type C, except weakness.

If you have been suffering from type B you would have been feeling very run down and tired, this is because you will have suffered all of the above symptoms. The type B virus is usually quite constant but it does occasionally alter just like the cold virus. A new strain can often cause small outbreaks such as the Swine Flu outbreak.

The type A version of influenza is the worst out of all three, not only do you suffer all the above symptoms, but it is highly unstable. It is very rare for a person to die from flu however a common complication of influenza is a bacterial chest infection. A bacterial chest infection can develop into a severe case of pneumonia. A course of antibiotics usually cures the bacterial infection, but itcan turn into a life-threatening situation, especially in the weak and elderly.

Anti-Influenza vaccines are available free of charge at the GP Surgery, but they only have a 60-70% success rate in preventing infection (NHS Choices, 2010). Individuals should have the vaccination every year and this should be just before the 'flu season' begins. Older people are recommended to have a vaccination because they are considered to be in the high risk category along with anyone suffering from respiratory or circulatory diseases (NHS Choices, 2010).

Antiviral medicationcan be prescribed by your GP, but only if you are in the high risk category and have flu like symptoms. Antiviral medication is not a cure for flu, but can help minimise the duration of the illness, provide relief for some of the symptoms, and can also reduce the potential for complications. The differenttypes of antiviral medication are Zanamivir (Relenza), Oseltamivir (Tamiflu) and Amantadine and these antiviralsare designed to stop the virus from multiplying within your body (NHS Choices, 2010).

A person suffering with influenza should ensure they get plenty of rest. They can also take painkillers, which will help to relieve aches and pains and also reduce any fever. Warm fluids can be drunk to help soothe a sore throat and inhaling steam can have a soothing effect on the lungs.


A disease is a medical condition that arises when something inhibits the body from functioning normally. There are two types of diseases: Non-infectious and Infectious. Non-infectious diseases are caused by microscopic organisms that invade the body, but do not or are not known to involve, infectious agents. Infectious diseases are caused by microscopic organisms that invade the body and can be transmitted from one person to another.

Infectious Diseases can be transmitted in various ways, such as:

  • Droplets - when an infected person coughs or sneezes, their droplet contains pathogens which will transmit the disease if breathed by an uninfected person
  • Dust - some diseases, like diphtheria and scarlet fever, can be spread by dust. Germs stick to the dust particles and float through the air. People can atchj the disease by breathing in the dust.
  • Touch - when an infected person touches an uninfected person, the pathogen can enter through the skin.
  • Cuts - Pathogens enter the body where the skin is cut by a contaminated object.
  • Food or water - pathogens in contaminated food and or water can enter the body through the soft gut wall.
  • Sexual intercourse - sexually transmitted diseases gain entry through the soft mucous membranes of the penis and vagina.
  • Insects - blood sucking insects inject their mouthparts through the skin, and can transmit pathogens that they sucked from an infected person

Our bodies are protected from germs in many ways. Some of the ways arise from the body's own defence mechanisms and others are man-made.

Man-made solutions include:

  • Immunisation - Immunisation is a vaccine that stimulates the immune system to produce antibodies that fight against the disease. Immunisation also helps to teach the immune system how to produce the
    appropriate antibodies quickly. After immunisation, if the individual comes into contact with the disease, their immune system will recognise it and will instantly begin to produce the antibodies required to fight it.
  • Ready-made antibodies - this is where an individual is given an injection of antibodies that are made by another organism. This is useful in circumstances where the body would not have enough time to produce
    antibodies of its own. For example, if you are bitten by a snake, you may be given an injection of ready-made anti-venom, which is an antibody.
  • Antibiotics - Antibiotics is a substance that destroys bacteria. They can be either synthetically produced or produced from micro-organisms, for example penicillin is formed from a bacterial mould.

The body's own defence mechanisms:

The body has its own defence mechanisms, known as the barriers to infection, which are features of the human body that prevent micro-organisms from entering the body.

  • Skin - the skin forms a physical barrier that prevents most foreign invaders from entering the body. The sebaceous gland secretes lactic acid and lysozymes, together these two substances slow down bacterial
    growth by breaking down their cell walls.
  • Mucous - contains lysozyme which inhibits the growth of and destroys bacteria.
  • Cilia - the cilia are microscopic hair-like projections on the cell, which trap foreign invaders that manage to enter the body
  • Tears - the tear fluid lubricates and protects the eyes from foreign matter and infection
  • Stomach Acid - hydrochloric acid in the stomach destroys micro-organisms that may have been ingested in the food. Mucin is a substance that coats the stomach, which protects it from the effects of the acid.


Antibiotics are chemical compounds, which are safe and effective drugs that treat bacterial infections. They can be divided into two separate groups, bactericidal and bacteriostatic. Bactericidal antibiotics simply destroy the micro-organisms they target, whereas bacteriostatic antibiotics do not destroy the micro-organism but prevents them from multiplying but inhibiting their growth and reproduction, enabling the immune system to overcome the infectious bacteria. The one thing that all the different types of antibiotic have in common is their definition, which is "an antibiotic is a substance which is produced by one type of micro-organism which kills or stops the growth of another" (Indge, 2003).

Bacteria destroying compounds can be found in some plants, insects and amphibians. Antibiotics are mainly all produced from microorganisms, usually in the form of bacteria or fungi, although this is now becoming complicated because chemists can alter the structure of naturally found bacteria to increase its effectiveness. An example of naturally found bacteria that has been changed (semi-synthetic bacteria) is the antibiotic penicillin. Penicillin is now used worldwide for many different bacterial infections.

In 1928 a bacteriologist called Alexander Fleming, made a discovery which would ultimately make him the founder of modern day antibiotics. One day Fleming forgot to cover one of his dishes in which he was growing bacteria and the next day he found that the bacterial colonies inside this one dish had been killed. After a lot of searching he found that his bacteria had been destroyed by a blue/green mould. He identified this mould to be penicillium notatum, which is a mould that grows on the surface of fruit, and he realised that this mould must have produced a chemical substance which destroyed the bacteria. From this realisation, Fleming discovered a bacterial fighting substance which is today known as Penicillin. Although Fleming discovered this substance, it was not until 12 years later two Oxford scientists, Ernst Chain and Howard Florey, managed to isolate and purify the penicillin and thus discovered the importance of this antibiotic (MacKean, 2000).

Since the 1940's antibiotics have saved millions of lives, they are mostly harmless to humans as the biochemical reactions that they target in bacteria are different to that in animals. Even though antibiotics can be very effective in destroying bacteria, and that the war against bacteria seemed to have been won, in the 1960s they found that the bacteria were finding a way to fight back. Some strains of bacteria have arisen that are causing infections that are very difficult to treat, as they no longer respond to an antibiotic for example, MRSA (methicillin-resistant Staphylococcus aureus). This has become a topic that has caused a lot of controversy and panic, and is known as antibiotic resistance.

Bacterial infections are becoming more and more common and have become a major health threat to the population. It will also become the biggest heath threat if nothing is done about antibiotic-resistant bacteria. Antibiotics are no longer considered to be the super drug that they used to be simply because, even though the majority of bacteria are destroyed by an antibiotic, it takes just one antibiotic resistant bacterium to have a gene mutation that will enable it to survive and replicate with other germs. This resistance would then be passed on to the antibiotic. Bacteria exchange antibiotic resistant genes by passing plasmids that contain the genes, from one bacterium to another.

A bacterium becomes resistant to an antibiotic when one of its alleles becomes altered. There are four ways that this can lead to resistance.

The first is where the altered allele occurs in the gene which codes for the protein that is a target for the antibiotic. If the altered protein is a different shape, it can no longer bond to the antibiotic. Thus if there is no binding, the antibiotic can have no effect on the bacterium.

The second mechanism is where antibiotic-resistant bacteria stop the antibiotic reaching its target molecule inside the cell, by either preventing the antibiotic from entering the cell, or by pumping it out faster than it can get in. This can happen if the mutation occurs in an allele that codes for a membrane receptor or transport protein.

The third mechanism is where the target of an antibiotic is an enzyme. Bacteria that produce an alternative version of the enzyme can bypass the antibiotic. This enzyme would still carry out the same function as would originally, but the antibiotic does not have any affect on it. Having both forms of the same enzyme gives the bacteria a distinct advantage, as it can survive equally well whether or not the antibiotic is present.

The fourth mechanism is where a mutation might enable the bacterium to produce an altered enzyme that is then able to react with the antibiotic and disable it.

The worldwide issue over the overuse of antibiotics is the foundation and plays the main role in the serious situation of antibiotic resistance. Individuals need to start realising that our bodies contain good bacteria and that this good bacteria has the capabilities and is strong enough to fight off other bacteria, such as pathogenic bacteria. The overuse of these antibiotics are primarily a result from GP's over prescribing antibiotics, or by prescribing antibiotics when they are not required. Other issues that contribute to the overuse of antibiotics is that many individuals do not complete the course of antibiotics of which they were supplied. These individuals simply stop taking their prescribed antibiotics when they start to feel well again, this allows the bacteria, with even a slight resistance, to re-establish an infection.

Farmers are also making a contribution to the resistance of antibiotics, by overusing agricultural antibiotics, as there are no limits to the amount that they can use. The bacteria that are being used on animals and plants, for growing purposes, are becoming harmful to the humans as the food that they are eating is becoming infected with these, such as Salmonella.

In conclusion, it can be said that antibiotic-resistant bacteria have an impact on the environment as a whole. Antibiotic-resistance cannot be overturned without the help of the patients, GP's, farmers, and the health officials. Each and every day, more and more bacteria are becoming resistant to antibiotics and if this issue is not sorted, the infections that result from these bacteria could lead to death.

  • MacKean, D., (2000) Life Study - A Textbook of Biology. UK: John Murray (Publishers) Ltd.