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The immune system

The immune system is the body’s natural defence against infection and disease. The human body is protected by 2 main types of immunity.

  • Innate immunity is the built-in protection present at birth that defends us from disease.
    • Skin and other tissues lining our body (such as the mucosal layer in the nose and mouth) form a barrier to keep invaders out.
    • White blood cells respond to invaders, such as bacteria and viruses, and remove or destroy them.
  • Acquired immunity is the protection the body develops when it is exposed to certain diseases.
    • The first time the body encounters a bacterium, virus or other antigen, it learns to recognize its antigen. The next time it invades, the immune system remembers it, has a stronger reaction to it and can fight it off quicker.
    • Vaccines are based on acquired immunity. They contain small amounts of protein (antigen) from a disease-causing micro-organism. If that micro-organism ever invades the body, the immune system will recognize and fight it.

Structure of the immune system

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 system.

  • Adenoids are actually a single, small mass of lymphatic tissue at the back of the nose. They contain lymphocytes.
  • Tonsils are small masses of lymphatic tissue in the throat that contain lymphocytes.
  • The thymus is a small gland in the centre of the chest behind the breastbone (sternum). It is where T cells (T lymphocytes) mature and multiply.
  • The spleen is an organ on the upper-left side of the abdomen near the stomach. It makes lymphocytes, stores blood cells, filters the blood and destroys old blood cells.
  • Lymph nodes are small, bean-shaped masses of lymphatic tissue along the lymph vessels (tubes through which lymph fluid travels in the body). They are located throughout the body. Many lymph nodes are grouped together in the neck, armpits, abdomen and groin. They store lymphocytes and filter bacteria and foreign substances (including cancer cells) from lymph fluid.
  • Other lymphatic tissues are located in other parts of the body, including the appendix and small intestine (Peyer’s patches). They function like lymph nodes.
  • Bone marrow is the soft, spongy tissue in the centre of some bones. It is where immature blood cells develop into mature blood cells. These include white blood cells (WBCs), which help fight infections.

Cells and substances of the immune system

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.

Types of lymphocytes
Name of lymphocyteDescription

B cell

(B lymphocyte)

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 cell

(T lymphocyte)

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 cytokine), which activate other cells and substances.

There are 3 types of T cells:

  • Helper T cells stimulate B cells to make antibodies and help killer T cells work better.
  • Killer T cells (cytotoxic T cells) destroy cancer cells and cells containing foreign substances, such as bacteria or viruses.
  • Suppressor T cells make substances that help turn off the immune system response.

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.

Types of granulocytes
Name of granulocyteDescription


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 and antigen-presenting cells

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.

Types of APCs
Name of APCDescription


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:

  • Monocytes are phagocytes that circulate in the bloodstream.
  • Macrophages are phagocytes that develop from monocytes that have moved from the blood into tissues or organs, such as the spleen, bone marrow and liver. Macrophages produce cytokines.

dendritic cell

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 (immunoglobulins)

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:

  • Antibodies bind to antigens (much like a key fitting into a lock) to mark them for destruction.
  • Antibodies identify antigens as foreign so that other immune system cells can destroy them.

Types of antibodies (immunoglobulins)
Name of antibodyDescription

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:

  • colony-stimulating factors
  • interferons
  • interleukins
  • tumour necrosis factor (TNF)

Complement system

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:

  • helps antibodies in the body destroy invaders
  • attracts phagocytes to bacteria or other invaders
  • helps kill bacteria or foreign invaders

lymphatic system

The group of tissues and organs that produce and store cells that fight infection and diseases.

The lymphatic system includes the adenoids, tonsils, spleen, thymus, lymph nodes, lymph vessels and bone marrow.

Also called the lymph system.


A type of white blood cell that fights viruses, bacteria, foreign substances or abnormal cells (including cancer cells).

The 3 types of lymphocytes are B cells, T cells and natural killer (NK) cells.

lymph fluid

A clear, yellowish fluid that contains nutrients, lymphocytes (a type of white blood cell that fights germs, foreign substances or cancer cells) and antibodies. Lymph fluid circulates throughout the body in lymph vessels and bathes body tissues.

Also called lymph or lymphatic fluid.

Peyer’s patch

One of several clusters of lymphatic tissue in the lining of the ileum (lower part of the small intestine) that works like a lymph node to recognize and fight infections in the intestine.

Also called aggregated lymphoid nodules.

white blood cell (WBC)

A type of blood cell that helps the body fight infection and diseases.

White blood cells are made in the bone marrow and are found in the blood and lymphatic tissue. They play an important role in immune response (the immune system’s reaction to the presence of foreign substances in the body).

Also called leukocyte.


A 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).

Cytokines are produced by the body or can be made in a lab.


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