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The immune system is the body’s natural defence against infection and disease. The human body is protected by 2 main types of immunity.
When the body detects something foreign, several kinds of cells and substances go into action to protect us. This is the immune response.
The immune system is made up of cells and organs that work together to respond to infections, abnormal cells and foreign invaders, as well as transplanted tissues. Immune system cells circulate through the bloodstream or the lymphatic systemlymphatic systemThe group of tissues and organs that produce and store cells that fight infection and diseases..
The immune system is made up of different types of cells and substances. White blood cells are the most important cells of the immune system. Different types of white blood cells (lymphocytes and granulocytes) work with other parts of the immune system in slightly different ways. The lymphatic system and other substances also play important roles in the immune system response. These other substances include antigens, antigen-presenting cells, antibodies (immunoglobulins), cytokines and the complement system.
Lymphocytes are a type of white blood cell. They make antibodies and other substances that fight infection and disease. Lymphocytes are found in the blood and throughout the body in lymphatic tissue. They attack viruses, bacteria and other micro-organisms.
|Name of lymphocyte||Description|
B cells mature in the bone marrow, and then move into the lymph nodes and spleen.
B cells make proteins (antibodies) in response to foreign substances (antigens). Antibodies recognize and bind to antigens, marking them so other immune system cells can find and destroy them. Each type of antibody can bind to only a specific type of antigen.
T cells mature in the thymus, and then gather in lymph nodes and the spleen.
T cells directly attack foreign invaders and cancer cells. They direct and control the body’s immune response by signalling other immune system cells to areas where they are needed. They mainly work by making lymphokines (a type of cytokinecytokineA substance made by cells of the immune system. Cytokines allow immune system cells to communicate with each other and thus help carry out the body’s immune response (the immune system’s reaction to the presence of foreign substances in the body).), which activate other cells and substances.
There are 3 types of T cells:
natural killer (NK) cell
NK cells are made in the bone marrow.
NK cells attach themselves to cells infected with micro-organisms (such as viruses or bacteria) and to cancer cells. Once attached, they produce chemicals (cytokines) that damage and kill the cells.
Granulocytes are white blood cells that fight infection. They are also called polymorphonuclear leukocytes (PMNs). These white blood cells contain tiny grains (granules) filled with chemicals. When these chemicals are released, they help to destroy micro-organisms and contribute to inflammatory and allergic responses.
|Name of granulocyte||Description|
Neutrophils eat (ingest) and destroy bacteria, fungi, viruses and other foreign cells. This process is called phagocytosis.
They are the body’s primary defence against harmful bacteria.
Eosinophils kill parasites.
They play a role in allergic reactions.
Basophils release histamine and play a role in allergic reactions.
They produce substances that attract neutrophils and eosinophils to an area to help fight infection.
Antigens are substances on the surface of cells. When T cells or antibodies recognize antigens, they trigger an immune response. Antigens can be anything the body recognizes as foreign, including bacteria, viruses, cancer cells or other invaders that can cause infection or disease.
Antigen-presenting cells (APCs) surround the foreign invaders. Then the APCs show (present) the antigens from these foreign invaders to T cells and B cells. The T cells and B cells can then recognize and respond to the invaders.
|Name of APC||Description|
Phagocytes are large white blood cells. They surround, swallow and digest foreign substances and damaged cells. (This process is called phagocytosis.) Once these foreign substances are broken down, it is easier for T cells and B cells to recognize and destroy them.
There are 2 types of phagocytes:
Dendritic cells are a type of white blood cell found in lymph nodes, skin and some organs.
They ingest and break an antigen into pieces, then stimulate T cells to destroy the antigen.
Antibodies are proteins made by B cells. They fight infection and defend the body against harmful foreign substances by recognizing and binding to antigens. An antibody has 2 parts. The top part (variable region) is different from antibody to antibody because it is designed to bind to a specific antigen. The bottom part is constant and determines the type of antibody (the immunoglobulin class) produced.
Antibodies travel freely in the blood. They destroy antigens in 2 ways:
|Name of antibody||Description|
immunoglobulin G (IgG)
IgG is the most common immunoglobulin.
It covers foreign substances so other components of the immune system can easily recognize them.
immunoglobulin M (IgM)
IgM is very effective at killing bacteria.
immunoglobulin A (IgA)
IgA concentrates in body fluids, such as tears, saliva and secretions of the respiratory and digestive tract.
It defends against invading micro-organisms.
immunoglobulin E (IgE)
IgE protects against parasitic infections.
It is responsible for allergic reactions.
immunoglobulin D (IgD)
Its function is not well understood.
Cytokines are chemical substances made by many different cells in the immune system. They are the messengers of the immune system. They allow immune cells to communicate with each other and they help carry out the defence response of the immune system.
Some cytokines are used to stimulate the immune system to attack cancer cells:
The complement system is made up of several proteins in the bloodstream. These proteins circulate in the bloodstream until they are activated by antibodies. (This process is called the complement cascade.) The complement system:
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