The immune system is an intricate network of organs, cells and proteins which are responsible for defending the body against infection. Like a logbook, the immune system keeps a perfect record of every germ (microbe) it has ever encountered so it can recognise the microbe quickly if it ever encounters it again. Some of the body parts which have a role in the immune system include white blood cells, the spleen and bone marrow.

Activation of the immune system

Anything which the body does not recognise as its own can activate the immune system. These are called antigens. For example, antigens can be found on bacteria, fungi, and viruses. Body cells also have proteins on their surfaces, but these do not trigger the immune system as the body recognises them as self. If the body mistakes its own body cells as foreign, it will attack them in an autoimmune response.

There are two types of immune system

The first subsystem of the immune system is the innate immune response. This is non-specific and provides a more generalised defence against germs. The innate immune system targets invaders using cells called natural killer cells and phagocytes. The innate immune system will come into play when germs have entered the body through the skin or digestive system. The innate immune system is present at birth. The adaptive immune system is the second type of response. This system creates antibodies which can specifically fight particular germs, which the body has encountered before. One key feature of the adaptive immune system is that it can adapt to learn how to neutralise bacteria and viruses which are developing over time. The adaptive immune system develops as the body is exposed to germs or chemicals released by germs.

Cells involved in an immune response

Phagocytes are key cells involved in an immune response. They work by surrounding and absorbing the microbe to eliminate them. There are five types of phagocytes: neutrophils, macrophages and dendritic cells, monocytes, and mast cells. If inflammation is present in the body, the neutrophils are responsible for an early response where they kill the microbe. However, in killing the microbe, they sacrifice themselves, with each one dying after completing their job. The macrophages are the ‘cleaners’ which remove any dead microbes or neutrophils. Dendritic cells are the ones who activate the immune response, which is like mast cells trigger an immune response when they come in contact with an antigen.
Lymphocytes are the cells responsible for allowing the body to remember previous invaders. They originate from the bone marrow. If they stay in the bone marrow they will become B lymphocytes, but if they travel to the thymus, they will become T lymphocytes. B lymphocytes recognise the antigen and start releasing antibodies (which attach to the antigens).

T lymphocytes are split into two categories. Helper T cells are responsible for controlling immune response. They can communicate with other cells as well as tell B cells to release more antibodies. The second type of T cell is the killer T cell. They recognise elements of a virus on infected cells and go on to destroy the infected cells.

What is immunity?

Immunity means we can resist infection or toxin using antibodies or white blood cells. With each exposure to a microbe, the immune system becomes stronger. Therefore, by adulthood, the majority of adults have been exposed to a range of microbes, meaning they have developed some level of immunity. When an antibody has been generated by the body, it stores a copy of the antibody for future use. Vaccination uses this principle. There is a natural level of immunity in the body called innate immunity. This is the level of immunity that a person is born with, ready to defend the body from birth.

What happens if the immune system goes wrong?

In cases where the immune system is not working as expected, the person could suffer from an autoimmune condition, hypersensitivity, or immunodeficiencies. Immunodeficiencies can be present from birth or develop over time. Hypersensitivity is where the immune system reacts too strongly to everyday substances. Examples of hypersensitivity include asthma and eczema. Autoimmunity occurs when healthy cells are targeted by the body instead of microbes. Type one diabetes is an example of this as cells of the pancreas are destroyed rendering them unable to produce insulin. Other examples include celiac disease and rheumatoid arthritis.