The immune system is a network of cells, tissues, and organs that work together to protect the body from infection. The human body provides an ideal environment for many microbes, such as viruses, bacteria, fungi, and parasites, and the immune system prevents and limits their entry and growth to maintain optimal health.
How does the immune system work?
The first line of defence is your body's skin and mucous membranes.
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.
Some disorders of the immune system
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HIV and AIDS - when the immune system is weakened
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Lymphoma, myeloma and leukaemia - cancers of cells which are part of the immune system
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Allergy - when the immune system overacts
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Antibody and antigen tests - may be done to help identify certain infections and some other disorders