The Defence forces
The immune system has developed over the millenia as a combination of defences for the body against invading organisms – ‘antigens’. Without it we would not survive even the simplest of infection. This vast and complex system allows for the great variety in antigen, which it may need to fight. As just one example there are 100,000,000 possible varieties of antibody. The method by which different types of antigen are recognised and dealt with also shows a wide variation. For example, whether the pathogen is a virus, bacteria, fungus or protozoa, intracellular or extracellular will make a difference to the mechanism of immunity.
The diagram below explains what the Immune System is all about.
Cells
The white blood cells involved in innate immunity – the granulocytes including neutrophils basophils and eosinophils – are produced, in the adult, by the myeloid cells in bone marrow especially in the sternum, ribs, pelvis and vertebrae. The liver is also involved in granulocyte production in the newborn but this ability decreases, although it can occur again later in adult life if needed.
The lymphocytes are produced in lymphoid cells present in bone marrow. Future B cells continue to mature within the bone marrow but others travel to the thymus where they develop into T cells. All then move on to secondary lymphoid organs such as the spleen, lymph nodes and mucosal associated lymphoid tissue (MALT), where they gather in B or T cell areas. From this secondary site they are ready be sent to do their job of fighting invading antigens.
Invading antigens
As a generalisation, the following is one way in which the body deals with an invading antigen. The innate immune system in the first line of defence. For example, a bacterial colony present on the surface of the skin first has to get through the innate immune defences. It is unable to pass the through the barrier of the skin surface and is prevented from multiplying to any great degree by the presence of fats in the sebum.
An injury to the skin such as a cut may break that preventative surface and the bacteria enter the underlying tissue, whilst some will make it to the blood stream. The trauma and damage to cells by the cut will release a range of chemicals. These cause local blood vessel dilation, bring more blood to the area to help with the healing process, the leaking of fluid from blood vessels into the tissue causing swelling, and send a signal to attract inflammatory cells to the site. These cells, including neutrophils, leak into the tissue from the blood, then recognise foreign particles and eat them in a process called phagocytosis as well as releasing enzymes to break down the injured area before it can be rebuilt. Some of the bacteria may escape this but will have activated the release of chemicals such as complement, by other cells, which may dissolve the outer skin of the bacterial cell, thus killing it.
Antibodies
This will also play a part by both antibody-mediated immunity and cell-mediated immunity. Antibodies are released by B-lymphocytes, and stick to the wall of the bacterium. The antibody prevents the cell sticking to the host cells and also neutralises any toxic chemicals released by the bacterium. It then activates other chemicals in the blood such as the complement system and prepares the bacterium for attack by those chemicals. Then pieces of the bacterium are connected to the MHC part of antigen-presenting cells and presented to T cells, which can call on macrophages to digest the antigen pieces.
The enemy within
The immune system is a very complicated process best considered in small parts. A mixture of barriers, fighting cells which eat anything ‘foreign’, chemicals which stick to anything ‘foreign’, and cells which recognise the ‘foreign’ protein ‘fingerprint’ on the cell wall of any invading micro-organism and either release chemical warfare of their own or call on further fighting cells to digest the invader. Unfortunately for us the system doesn’t always work or is overwhelmed by the invaders and we may succumb to infection.
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