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Basic immune system facts

What is the immune system?

The immune system is a network of cells, tissues, and organs that together defend your body against attacks by external harmful “things”.

These “things” mainly include microbes (germs): extremely small organisms which cause infection. These might be bacteria, viruses, parasites, or fungi.
A common name for these microorganisms that can cause disease is pathogens.

The human body provides an ideal environment for many microbes – and they always try to “get into” it. It is your immune (protective, defensive) system which keeps them out. In case microbes are inside the body, your immune system will seek for them and destroy them.

The protective system of human body is an amazing and very complex mechanism. It works by recognizing and remembering millions of different enemies, and producing means for fighting each and every one of them.

How your immune system works

The secret to this ability of your body to protect itself against external “invaders” is a sophisticated communications network. Millions of cells, organized into sets and subsets, pass information between them. Once immune cells receive the alarm, they adapt to this change and begin producing powerful chemicals. These substances allow the cells to control their own growth and behavior, “remember” their fellows, and direct “soldier” cells to trouble spots.

The foundation to a healthy immune system is its unique ability to distinguish between the body’s own cells and foreign cells. The body’s immune defenses usually coexist with cells that carry distinctive “self” marker molecules. But when the system encounters cells carrying "nonself", foreign marks, an attack is launched.

Anything that can prompt an immune response is called an antigen. An antigen can be a microbe such as a virus, or even a part of a microbe. Not only harmful microbes can be marked as “foreign” and act as antigens, but also tissues or cells from another person. This is why transplanted organs may be rejected.

In abnormal situations, "self" cells can be mistaken for "nonself", which results in an attack against the body’s own cells or tissues. This situation is called an autoimmune disease. Some forms of arthritis and diabetes are autoimmune diseases.

In other cases, the system can respond to a seemingly harmless foreign substance such as ragweed pollen. This results in an allergy, and this kind of antigen is called an allergen.

The structure of the immune system

The immune system organs are located throughout the body. These organs are called lymphoid organs because they are home to lymphocytes, white blood cells that are the key players in the process of protecting your body from diseases.
  • Bone marrow is the source of all blood cells, including white blood cells
  • The thymus is an organ that lies behind the breastbone; lymphocytes known as T lymphocytes, or just “T cells,” are formed in the thymus.
  • Lymph nodes are spread along the lymphatic vessels, with clusters in the neck, armpits, abdomen, and groin. Each lymph node contains specialized compartments where immune cells congregate, and where they can encounter antigens.
  • Spleen is a flattened organ at the upper left of the abdomen. Like the lymph nodes, the spleen has specialized sections where immune cells gather – and where they confront antigens.
  • Tonsils, adenoids, and appendix are also considered as lymphoid tissues. They are located at the body “gateways”: airways and digestive tract.

Cells of the immune system

Normally, only a few of each kind of the different cells needed to recognize millions of possible enemies are kept in your body. When an antigen appears, those few matching cells multiply into many identical cells which start to fight the “enemy”.

All immune cells begin as immature stem cells in the bone marrow. These undeveloped cells respond to different signals to grow into specific immune cell types.

There are many types of cells of the immune system. The major types are:

  • B cells (B Lymphocytes)
    B cells work mainly by producing substances called antibodies into the body’s fluids. Antibody is a special molecule which can identify some specific antigen, and attach to it, thus marking it for destruction by other immune cells.

  • T cells
    There are two types of T cells.
    Cells of one type are called Helper T cells. Their role is to coordinate immune responses by communicating with other cells.
    Second type is Killer T cells. These cells can directly attack and destroy cells which carry “foreign” signs. For example, T cells can recognize viruses hiding inside a cell, and then to kill this infected cell.

  • Natural killer (NK) cells
    NK cells are another kind of lethal white cell (lymphocytes). Like killer T cells, NK cells are armed with granules filled with potent chemicals. NK cells recognize cells lacking body cells “self” identification. They have the potential to attack many types of foreign cells.

  • Phagocytes
    Phagocytes are large white cells that can literally swallow and digest microbes and other foreign particles. Phagocytes in the blood are called monocytes. Monocytes in body tissues transform into macrophages, which serve as cleaners from the “cell garbage”.


Immunity is an ability of organism to stand against particular antigen (disease).

It was noted that people recovered from particular diseases do not get them again. This is because some of the activated T and B cells become memory cells. This means, the next time this person meets up with the same antigen, the defensive system of the body is set to fight and to destroy it.

There are different ways how one’s body can get immunity. Naturally it can happen when you get antibodies as a result of decease. Another natural way of “immunization” is the way all babies get it - from their mothers. Babies who are nursed can also receive some antibodies from breast milk.

An immune response can be triggered not only by infection but also by immunization with vaccines - vaccination. Vaccines contain microorganisms — or parts of microorganisms — that have been treated such that they only can provoke an immune response but not full-blown disease.

Immune system diseases and disorders

  • Allergy
    When the immune system responds to a “false alarm” – you get allergy. An allergy happens when a harmless matter – pollen, or dust, or some food, like peanuts – is identified by the immune system as a threat, and is attacked.

  • Autoimmune Diseases
    Sometimes the immune system’s recognition ability breaks down, and it begins manufacturing T cells and antibodies directed against its own cells and organs. Conditions like diabetes, or rheumatoid arthritis are caused by “misguided” T cells that attack body organs (pancreas in case of diabetes, for example).

    The root cause of autoimmune disorders is yet to be discovered. Among possible contributing factors are viruses, certain drugs, and sunlight, all of which may damage or alter normal body cells.

  • Immunodeficiency Disorders
    When the immune system is missing one or more of its components, the result is an immunodeficiency disorder. Immunodeficiency disorders can be inherited, acquired through infection, or appear as a side effect of drugs such as those used to treat people with cancer or those who have received transplants.

    Immune responses can be depressed by blood transfusions, surgery, malnutrition, smoking, and stress. By far the most known immunodeficiency disorder is AIDS: Acquired immune deficiency syndrome. AIDS is caused by a virus (HIV) that infects immune cells. HIV can destroy or disable vital T cells, making the way for a variety of diseases.

Adapted from “Understanding the Immune System” by National Institute of Allergy and Infectious Diseases. Original article is much more detailed.
Many excellent pictures of the immune system may be found in this PDF document. The file is big, downloading may take some time.

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