The immune system consists of organs, cells and the secreted chemicals that fight infection (microbes). The main components of the immune system include: white blood cells, antibodies, the complement system, the lymphatic system, the spleen, the thymus, and the bone marrow. All these are involved actively in fighting the infection.

The Immune System And Microbial Infection

The immune system has the memory of every microbe or foreign particle it has encountered, in the white blood cells called B- and T-lymphocytes commonly known as memory cells. If the body recognizes the same microbe in future, it will destroy the microbe quickly before it can multiply and cause disease.

Some infections, like the flu and the common cold, are fought like this many times because many different viruses or strains of the same type of virus will cause these illnesses. Hence catching a cold or flu from one virus does not make one immune against the others.

Different Parts Of The Immune System

The main parts of the immune system are:

  • White blood cells
  • Antibodies
  • Complement system
  • Lymphatic system
  • Spleen
  • Bone marrow

White Blood Cells

White blood cells are very important key players of the immune system. They are made in the bone marrow and form part of the lymphatic system. 

White blood cells travel through blood and tissue throughout the body, in search of foreign invaders (microbes) such as bacteria, viruses, parasites and fungi. When they encounter them, they launch an immune attack and neutralise them.. 

White blood cells include lymphocytes (such as B-cells, T-cells and natural killer cells), and many other types of immune cells.


Antibodies help the body in fighting microbes or the toxic molecules produced by them. The antibodies recognise the substances called antigens present on the surface of the microbe, or the molecules they produce. These antigens or molecules mark the microbe or toxin as a foreign particle and attack them involving many cells, proteins and chemicals.

Complement System

This includes the proteins secreted whose actions complement the work done by antibodies.

Lymphatic System

The lymphatic system is a network of delicate tubes distributed throughout the body. The main functions of lymphatic system include:

  • managing the fluid levels in the body
  • reacting to bacteria
  • dealing with cancer cells 
  • dealing with cell products that otherwise would result in disease or disorders
  • absorbing some of the fats in our diet from the intestine.

The lymphatic system consists of:

  • Lymph nodes (also called lymph glands) — which trap microbes
  • Lymph vessels — tubes that carry lymph, the colourless fluid that bathes the body’s tissues and contains infection-fighting white blood cells
  • White blood cells (lymphocytes).
  • The lymph nodes and other lymphatic structures like the spleen and thymus contain white blood cells called lymphocytes. They rapidly multiply and release antibodies in response to the infections from bacteria, viruses, and a range of other stimuli from dead or dying cells and also abnormally behaving cells like cancer cells.


The spleen is a blood-filtering organ that filters microbes and destroys old or damaged red blood cells. It also makes some components of the immune system like antibodies and lymphocytes.

Bone Marrow

Bone marrow is a spongy tissue found inside the bones. It is involved in the production of red blood cells, the white blood cells, and the platelets.


The thymus filters and monitors the blood content. It is involved in the production of the white blood cells called T-lymphocytes.

Other Defence Mechanisms Of The Body

Along with immune system, body contains other ways of protection against pathogens like:

  • skin– a waterproof barrier which secretes the oil with bacteria-killing properties
  • lungs– mucous in the lungs (phlegm) traps foreign particles, and small hair like cilia waves the mucous upwards causing a person to cough to remove the particles from the body.
  • digestive tract– the mucous lining  antibodies, and the stomach acid kill most of the microbes.
  • other defences– body fluids like skin oil, saliva and tears all contain anti-microbial enzymes helping to reduce the risk of infection. The constant flushing of the urinary tract and the bowel also helps in removing microbes or toxins out of the body.

Fever- An Immune System Response

A rise in body temperature is a result of some infections, which is actually an immune system response. A rise in temperature will kill some microbes and also triggers the body’s repair process.

Common Disorders Of The Immune System

Sometimes the immune system could be overactive or underactive.

Overactivity of the immune system is caused due to:

  • Allergic diseases– here the immune system makes a strong response to allergens. Allergic diseases are very common and include allergies to foods, medications or stinging insects, anaphylaxis (life-threatening allergy), hay fever (allergic rhinitis), sinus disease, asthma, hives (urticaria), dermatitis and eczema
  • Autoimmune diseases– here the immune system recognizes the body’s own component as foreign and destroys it. They include multiple sclerosis, autoimmune thyroid disease, type 1 diabetes, systemic lupus erythematosus, rheumatoid arthritis and systemic vasculitis.

Underactivity of the immune system or immunodeficiency could:

  • Be inherited– like primary immunodeficiency diseases such as common variable immunodeficiency (CVID), x-linked severe combined immunodeficiency (SCID) and complement deficiencies
  • Arise as a result of medical treatment– caused due to medications such as corticosteroids or chemotherapy
  • Be caused by another disease– such as HIV/AIDS or certain types of cancer. 

Improper functioning of the immune system makes people vulnerable to infections which could be life threatening in severe cases.

People who have undergone organ transplantation are given immunosuppression drugs to prevent the body from attacking the transplanted organ.

Immunoglobulin (Antibodies) Therapy

Immunoglobulins (commonly known as antibodies) are used to treat people who are unable to produce them on their own, or whose antibodies do not work properly. The treatment is known as immunoglobulin therapy.

Immunisation (Vaccination)

Immunisation works by copying the body’s natural immune response. A vaccine which contains the attenuated form of the virus, bacterium or toxin is injected into the body. The body then makes antibodies to it.

When the vaccinated person is exposed to the actual virus, bacterium or toxin, the body recognises it and attacks it successfully. Vaccinations are available against many diseases, including measles and tetanus.

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