Human Immune Systems Against Infections Biology Essay

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One of the most vital parts of the human body is the immune system, responsible for protecting against numerous different infections and foreign invaders; such as bacteria, viruses and fungi. The immune system has many different lines of defence to equip itself to challenge the invaders. This could be considered as an attack on a castle, in this case the body is the castle and the immune system is the army protecting the castle. The castle is consistently at threat of invasion by antigens, whilst our army works hard to offer constant protection without our knowledge.

The first line of defence, is for the immune system to prevent the invaders, known as antigens from entering the castle. This is done through the use of physical and chemical barriers. The castle is protected by the skin and mucous membranes. The skin is capable of secreting sweat and sebum which combine together to form lysozymes which break down the cell wall of any bacteria which are found holding on to the outside of the castle, looking for a way to enter.

When the outer layer is intact, invaders are unable to enter the castle, however occasionally openings in the defence arrive; the nose, eyes, mouth, open wounds, etc. These are protected by the moat of the castle. Scabs are quick too form over open wounds sealing the entrance, like a row of soldiers blocking the way. An enzyme called lysozyme is found in tears, mucous and the vaginal opening protecting the openings from invasion. The mucus of the nose traps micro-organisms, preventing them from passing further and leaving them stranded in the moat. Any invaders which do manage to enter the tunnel, known as the respiratory tract, are quickly brought to a halt by the mucus and the soldiers known as cilia, before being marched back out the castle the way they entered. Any invaders who do sneak through and reach the stomach enter the cess pit of the castle, filled with acid, killing the invaders.

If the invasion of the first line of defence is successful they are then met by the second line of defence; the guards stood on the walls of the castle. This is referred to as a non-specific response. These deal quickly with the invaders and the army works in many different ways to see them off. The invaders are targeted, caught and then marched along via the lymphatic system, to the blood stream, rather like being marched to the corporal's office. Here the higher ranked soldiers; the white blood cells are in charge of dealing with them. The regiment of white blood cells responsible for this are the phagocytes, made from subdivisions; neurophiles and monocytes. "In this process, the white cell engulfs the pathogen, takes it into a vacuole inside the cytoplasm and then digests it with lytic enzymes." (Boyle et al, 1999, P.513). Due to the short life span of these soldiers, they are only capable of surviving for several days after killing the invaders and then form as pus on the surface of the skin.

The third line of defence targets specific pathogens. This are dealt with by specialist soldiers who are capable of dealing with specific immune responses. Lymphocytes are the major rank to deal with this attack. They are produced by the stem cells of the bone marrow. When the army gives them a call, they then leave the factory where they travel to the army base to prepare to be called upon to defend the body. There are two divisions based on their expertise; T cells and B cells.

The T cells, travel to their army head quarters in the thymus. Only a small amount of T cells reach the thymus as the body limit's the quantity to protect the body from damage from self-engulfing. Here they mature and become equipped for the attack. "The T cells are given T cell receptors, of which there are several types. The type of receptor received determines what type of T cell it will be, what its role is, and which cells it can interact with." (Virtual Medical centre, 2008). The other division, the B cells mature in the bone marrow before travelling to the lymph and distributed to the lymph nodes; the waiting room, where they wait until they are called upon.

B cells do not fight the invader directly, instead they are armed with weapons, known as antibodies. The antibodies then travel in the blood to the site of infection. "An antibody, or immunoglobulin, is a Y-shaped protein molecule that is made by a B lymphocyte in response to a particular antigen." (Boyle et al, 1999, P.515). There are five different types of antibodies produced; IgG, IgM, IgA, IgE and IgD. These are all immunoglobulin's, they have a similar main structure; the Ig, but a varying end structure, allowing the interaction to different antigens. These have various functions within the immune system and are capable of recognising millions of invaders and producing the relevant antibodies.

The antibody IgM is the largest antibody in size and distributed throughout the lymph and blood. This is the first type of antibody called in response to the infection, known as the primary response. Its structure is slightly different to that of other immunoglobulin's in that it is, "five normal immunoglobulin's joined at their bases." (Virtual Medical centre, 2008). This allows its response to be so effective.

When the specific antibody recognises the invader, it quickly multiplies, forming a clone of plasma cells, which secrete a large amount of the specific antibodies. "These receptors have the same shape as the antibody that will be produced by the B cell. The antibody molecules are able to combine specifically with antigens." (Indge et al, 2000, P.148). Once the infection has been tackled, the majority of the new B cells are no longer required and die.

The antibody IgG is found in all bodily fluids and is the only antibody which is capable to cross the placenta during pregnancy, offering similar immunity to the foetus. "Part of the IgG molecule is able to interact with lots of cells of the immune system, and so it has the ability to stimulate a very direct attack on anything that it recognises." (Virtual Medical centre, 2008). It is responsible for initiating the secondary response.

The secondary response is much quicker. Some of the B cells after tackling the infection, fail to die and instead are given a new role within the army as memory cells. This is important as some invaders, will attempt to enter the castle again. To prevent further infection and a siege on the castle, these memory cells are quickly called upon. "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, 1999, P.515). B cells are unable to carry out a response on their own though, they require the help of the T helper cells.

T cells are also specific in their role, only recognising specific invaders as part of a cell-mediated response. "They compromise about 75% of circulating lymphocytes. T cells are the predominant cell in the paracortical region of lymph nodes." (GP Notebook, 2009). There is a further three divisions; T killer cells, T helper cells and T suppressor cells. When the invaders are deep inside the castle, penetrating the body cells, these are urgently called upon. "The cell surface membranes of T cells contain specific receptors with particular shapes, similar to antibodies. However, these receptors do not recognise whole antigen molecules, unlike antibodies." (Taylor et al, 1997, P. 487).

"Helper T cells are by far the most common T cell. They make up more than three quarters of the T cell population." (Virtual Medical centre, 2009). These are responsible for controlling the immune response, rather like the army general, giving orders to his soldiers. They instruct the B cells to divide and produce antibodies, they instruct the other types of T cells to commence fighting, and finally call upon the macrophages to prepare to engulf the invaders.

Killer T cells, also known as cytotxic lymphocytes, are the main part of the army responsible for fighting. Once instructions are received from the general T helper cell, they begin their attack. They puncture holes in the cell wall of the invader, which ultimately results in the death of the invader. "The action of killer T cells stimulates an increase in macrophage activity to clear up the debris." (Aplastic Anaemia and Myelodysplasia Glossary, 2009).

Suppressor T cells are like a mediator. "When the immune response becomes excessive, or when the infection has been dealt with successfully, these T cells damp down the immune response." (Boyle et al, 1999, P.517). This prevents unneeded damage to the body and a waste of resources.

Therefore the structure and organisation of the immune system, is well developed into specific defences, allowing protection to the castle. If the first line is broken, the castle still has the protection of the second and third line of defence. It is only after all three lines of defence have been broken, that the castle becomes completely under siege, resulting in serious attack. This can be referred back to the body, in that once the different stages of the immune response fail, the body is then open to serious infection and diseases.


Aplastic Anaemia and Myelodysplasia Glossary (2009) T cells [online]. Available from: [Accessed 22nd November 2009]

BBC GCSE Bite size (2009) Pathogens [online]. Available from: [Accessed 22nd November 2009]

Boyle, M., Indge, B., Senior, K. (1999) Human Biology. Collins. P.513, 515, 517.

GP Notebook (2009) T Cells [online]. Available from: [Accessed 22nd November 2009]

Indge, B., Rowland, M., Baker, M. (2000) A New Introduction to Human Biology. Hodder & Stoughton. P.148.

Taylor, D.J., Green, N.P.O., Stout, G.W. (1997) Biological Science 2 - Systems, Maintenance and Change. Cambridge. Soper. P.487.

Virtual Medical Centre (2008) Acquired immune system (B cells and T cells) [online]. Available from: [Accessed 22nd November 2009]