The Immune System

This leaflet gives a brief overview of the immune system and how it works.

A. What is the immune system?
We are surrounded by millions of bacteria, viruses and other microbes (germs) that have the potential to enter our bodies and cause harm. The immune system is the body's defence against pathogens (disease-causing microbes). The immune system is made up of non-specialised defences such as skin and the acidic juice produced by your stomach. But it also has some highly specialised defences which give you immunity against (resistance to) particular pathogens. These defences are special white blood cells called lymphocytes. Other types of white blood cells play an important part in defending your body against infection.
The lymphatic system is also part of the immune system. The lymphatic system is made up of a network of vessels (tubes) which carry fluid called lymph. It contains specialised lymph tissue and all of the structures dedicated to the production of lymphocytes.

B. Where is the immune system found?
The immune system is generally divided into two parts. The first part is the defences you are born with. These form what are known as the innate system.
The second part of your immune system, known as immunity, develops as you grow. Your immunity gives you protection against specific pathogens. Not only can this system recognise particular pathogens, it also has a memory of this. This means that if you encounter a certain pathogen twice, your immune system recognises it the second time around. This usually means your body responds quicker to fight off the infection.

The innate system is found in many different places around the body. First line of defence is your skin. Skin forms a waterproof barrier that prevents pathogens from entering the body. Your body cavities, such as the nose and mouth, are lined with mucous membranes. Mucous membranes produce sticky mucus which can trap bacteria and other pathogens. Other fluids produced by the body help to protect your internal layers from invasion by pathogens. Gastric juice produced by the stomach has high acidity which helps to kill off many of the bacteria in food. Saliva washes pathogens off your teeth and helps to reduce the amount of bacteria and other pathogens in your mouth.
If bacteria or other pathogens manage to get through these initial defences, they encounter a second line of defence. Most of these defences are present in your blood, either as specialised white blood cells or as chemicals released by your cells and tissues.
The second part of your immune system, the part that gives you immunity, involves the activation of lymphocytes. This will be described later on. Lymphocytes are found in your blood and also in specialised lymph tissue such as lymph nodes, the spleen and the thymus.

C. How does the immune system work?
The first line of defence is your body's skin and mucous membranes, as mentioned above.
If pathogens manage to get through these barriers, they encounter special white blood cells present in your bloodstream. There are different types of white cells, called neutrophils (polymorphs), lymphocytes, eosinophils, monocytes, and basophils.
White blood cells travel in the bloodstream and react to different types of infection caused by bacteria, viruses or other pathogens. Neutrophils engulf bacteria and destroy them with special chemicals. Eosinophils and monocytes also work by swallowing up foreign particles in the body. Basophils help to intensify inflammation (swelling).
Inflammation is part of your body's immune response. Damage to your tissues causes the release of chemicals into the blood. These chemicals make blood vessels leaky, helping specialised white blood cells get to where they are needed. They also attract neutrophils and monocytes to the site of the injury, which helps to protect against a bacterial infection developing.
Lymphocytes have a variety of different functions. They attack viruses and other pathogens. They also make antibodies which help to destroy bacteria. Lymphocytes are divided into T cells and B cells. Bone marrow is the tissue found within the internal cavity of bones. It contains stem cells, which create B and T cells. B cells mature in the bone marrow whereas T cells mature in the thymus.These are the cells responsible for developing immunity to particular types of bacteria and virus.

B cells and T cells work in different ways. B cells produce antibodies. Antibodies are a special type of protein which attacks antigens. Antigens are like flags to our immune system. They usually identify a molecule as being foreign. They can be found on the surface of bacteria, but they can also be found on substances which don't cause disease - for example, in pollen, egg white or transplanted organs. An antigen is a chemical part of a molecule which generates an antibody response in the body. Literally it means antibody generator. One of the most amazing features of the immune system is that B cells can recognise millions of different antigens. B cells can recognise antigens that have never entered the body before, and even man-made molecules that don't exist in nature.
When a foreign particle enters the body, B cells recognise it, binding to the antigen on its surface. This activates the B cell which then changes into a plasma cell. The plasma cell makes antibodies specific to that antigen. Antibodies can immobilise bacteria, encourage other cells to 'eat' the pathogen and activate other immune defences. While some B cells become plasma cells, others don't. These cells live on as memory B cells that respond more vigorously should the same antigen invade the body again.

T cells directly attack the invading organism; however, they are not able to recognise antigens without the help of other cells. These cells process the antigen and then present them to T cells. T cells are very different from each other. When an antigen enters the body only a few T cells are able to recognise and bind to the antigen. While T cells also bind to antigens they need a second signal to become activated. Once activated, T cells get bigger and start to divide. These cells then target the invaders and release chemicals that destroy the pathogen. Like B cells, some of the T cells remain to form memory T cells. This allows the body to respond quickly if the same antigen enters the body.
The lymphatic system is a major part of the body's defence against infection. Lymph nodes are one of the components of this system. These are specialised structures which are found in lymph vessels. Lymph nodes are a filter for the lymph flowing through the vessels. They contain B and T cells which recognise bacteria and pathogens which have entered the lymph via the bloodstream. When foreign material is detected, other dedicated immune cells are recruited to the node to deal with the infection. This helps to prevent the infection from spreading throughout the body.

There are around 600 lymph nodes throughout the body, usually in groups. Large groups of lymph nodes are found in the groin (inguinal nodes), in the armpit (axillary nodes) and in the neck area (cervical nodes). In health they are pea-sized but if you develop an infection you may find that they become enlarged. This is due to an accumulation of lymphocytes and other cells of the immune system.

Lymphoid tissue helps to defend mucosal surfaces, such as the mouth and intestines, from infection. The tonsils, which are found in the back of the throat, often become enlarged in response to infection. These tissues help to trap bacteria and other pathogens and activate white blood cells.

The thymus is an important lymphatic organ. Found in front of your trachea (windpipe), its main role is to teach white blood cells to recognise our own cells. In order for the immune system to function properly, white blood cells must be able to discriminate between invading pathogens and the body's own cells. After T cells are produced in the bone marrow they migrate to the thymus. Here they are educated by the thymus to stop them from attacking our own cells. It is thought that some forms of autoimmune disease (where the body attacks itself) may be due to problems with this process. The thymus is at its largest during puberty, and gets smaller as you age.

The spleen is the largest single mass of lymphatic tissue in the body. Located close to the rib cage on the left side of the body, the spleen helps to filter the blood. It contains specialised tissue called white pulp. This contains white blood cells which respond to bacteria and other pathogens in a similar way to those in lymph nodes. Other tissue in the spleen, called red pulp, helps to remove damaged red blood cells and store platelets.