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Article: How does the immune system work

How does the immune system work - Novamed (Europe) ltd

How does the immune system work

The immune system is a complex network of cells, tissues, and organs that combat foreign invaders in the body. The immune system works on two levels - innate immunity and adaptive immunity.

The innate immune system is the first line of defence against invading bacteria, viruses, fungi, parasites or other potential pathogens. It includes physical barriers on surfaces of the body that block invaders from entering and natural germ-killing substances that kill invading microorganisms that do manage to breach those barriers.

The adaptive or acquired immune system is your second line of defence against invaders. It responds only to specific antigens from those foreign substances after they have been detected by specific antibodies called immunoglobulins circulating in the bloodstream. Recognised antigens are then destroyed with particular antibodies and destroyed cells and tissues.

The two systems interact, with the immune system getting important information from the environment and gaining new knowledge through contact with others that it protects. Anytime an individual encounters an antigen or newly formed immune cells that are foreign to them, they build up a record of that interaction in their memory through time and experience. This record is called a "memory".

An essential function of adaptive immunity is cell recognition by T-cells, which specialise in recognising "foreign" (self) peptides (antigens). The peptides can be made in response to a viral infection or during an allergic reaction following exposure to a particular antigen. There are two types of T-cells, helper T-cells and killer T-cells.

Helper T-cells release chemicals that trigger the other parts of the immune system to attack cells containing the peptides they recognise. Killer T-cells directly kill cells containing foreign peptides.

These are composed of B cells responsible for producing antibodies and plasma cells responsible for heavy antibody production. Their primary function is to identify foreign substances in your body and create antibodies against them for future destruction by killer T cells.
Antibodies are proteins produced by the B cells that will bind to antigens. They can bind with antigens both directly and through interactions with other proteins. Antibodies help the body recognise foreign invaders, which they then destroy.

Some antibodies are secreted into the bloodstream, but they don't immediately distinguish between "self" and "foreign". As a result, they can attach to self-tissues, causing them harm without being fully aware of it. This is known as the auto-immune response. Therefore, some antibodies will be meant to attack foreign substances but accidentally destroy or damage tissues in your own body, causing disease processes such as inflammatory bowel disease or arthritis.

Antibodies are formed in plasma B cells which are also included in the bone marrow. The antibodies themselves are released when specific receptors on the surface of the B cell bind to antigens in the body, and they will circulate in your bloodstream. They can distinguish between self and foreign substances by using a type of receptor known as CD20.

In humans, the absolute numbers of lymphocytes will vary widely among individuals, but they usually make up 20% to 40% of white blood cells circulating in the bloodstream. The white blood cells include different types of lymphocytes, or leukocytes, which are produced by stem cells found in the bone marrow. This accounts for the wide range of numbers found in different individuals.

The number of B cells in the bone marrow decreases with age, but this is compensated by increased plasma cells. The number of T-cells also decreases slightly with age, but the increase in B-cells again offsets this. It's thought that this decrease in both B-cells and T-cells could contribute to the declining function of the immune system with age, along with other factors which result in increased susceptibility to infection.

The secondary lymphoid tissues are lymph nodes, spleen and tonsils. These are continuous organs made up of many tiny lymphatic cells. They are also the tissues where immune responses are detected. Lymph nodes are masses of lymphatic tissue which collect fluid, proteins and other debris from tissues. They filter this fluid to remove any pathogens that may be present, then send white blood cells to destroy any detected. Splenic lymphocytes can see invading pathogens in the spleen itself, responding much faster than those found in lymph nodes which have to travel up to the organ from the bloodstream. The tonsils are collections of lymphatic tissue located at either side of your throat.

The spaces between the cells of these organs are filled with an extracellular matrix made up of collagen fibres called reticular fibres.

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