The Immune System : the Basics

The immune system is a host defense system made up of many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism’s own healthy tissue.  One of the recent key breakthroughs in medicine is based on a new understanding of the microbiota’s role in the body’s over-all defense system.

In many species, the immune system can be classified into subsystems, such as the innate immune system versus the adaptive immune system, or humoral immunity versus cell-mediated immunity. In humans, the blood–brain barrier, blood–cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral immune system from the neuroimmune system, which protects the brain.

Pathogens, like cancer cells and lyme’s bacteria, can rapidly evolve and adapt, and thereby avoid detection and neutralization by the immune system. However, multiple defense mechanisms have also evolved to recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess a rudimentary immune system in the form of enzymes that protect against bacteriophage infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants and invertebrates. These mechanisms include phagocytosis, antimicrobial peptides called defensins, and the complement system.

Jawed vertebrates, including humans, have even more sophisticated defense mechanisms,[1] including the ability to adapt over time to recognize specific pathogens more efficiently. Adaptive (or acquired) immunity creates immunological memory after an initial response to a specific pathogen, leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of vaccination.

Disorders of the immune system can result in autoimmune diseases, inflammatory diseases and cancer.[2] Immunodeficiency occurs when the immune system is less active than normal, resulting in recurring and life-threatening infections. In humans, immunodeficiency can either be the result of a genetic disease such as severe combined immunodeficiency, acquired conditions such as HIV/AIDS, or the use of immunosuppressive medication. In contrast, autoimmunity results from a hyperactive immune system attacking normal tissues as if they were foreign organisms. Common autoimmune diseases include Hashimoto’s thyroiditis, rheumatoid arthritis, diabetes mellitus type 1, and systemic lupus erythematosus. One of its mechanisms is molecular mimicry. Immunology covers the study of all aspects of the immune system.


1.  Beck G, Habitat GS (November 1996). “Immunity and the Invertebrates” (PDF). Scientific American. 275 (5): 60–66. Bibcode:1996SciAm.275e..60B. doi:10.1038/scientificamerican1196-60. Retrieved 1 January 2007.

2. O’Byrne KJ, Dalgleish AG (Aug 2001). “Chronic immune activation and inflammation as the cause of malignancy”. British Journal of Cancer. 85 (4): 473–83. doi:10.1054/bjoc.2001.1943. PMC 2364095. PMID 11506482.




The immune system protects the body against illness and infection caused by bacteria, viruses, fungi or parasites. It is a collection of reactions and responses that the body makes to damaged cells or infection. So it is sometimes called the immune response.

The immune system is important to cancer patients in many ways because:

•cancer can weaken the immune system

•cancer treatments may weaken the immune system

•the immune system may help to fight cancer

Cancer and treatments may weaken immunity

Cancer can weaken the immune system by spreading into the bone marrow. The bone marrow makes blood cells that help to fight infection. This happens most often in leukaemia or lymphoma, but it can happen with other cancers too. The cancer can stop the bone marrow from making so many blood cells.

Certain cancer treatments can temporarily weaken the immune system. This is because they can cause a drop in the number of white blood cells made in the bone marrow. Cancer treatments that are more likely to weaken the immune system are:


•targeted cancer drugs


•high dose of steroids

The immune system can help to fight cancer

Some cells of the immune system can recognise cancer cells as abnormal and kill them. Unfortunately, this may not be enough to get rid of a cancer altogether. But some new treatments aim to use the immune system to fight cancer. There are 2 main parts of the immune system:

•the protection we have from birth (in built immune protection)

•the protection we develop after having certain diseases (acquired immunity)

In built immune protection

This is also called innate immunity. These mechanisms are always ready and prepared to defend the body from infection. They can act immediately (or very quickly). This in built protection comes from:

•a barrier formed by the skin around the body

•the inner linings of the gut and lungs, which produce mucus and trap invading bacteria

•hairs that move the mucus and trapped bacteria out of the lungs

•stomach acid, which kills bacteria

•helpful bacteria growing in the bowel, which prevent other bacteria from taking over

•urine flow, which flushes bacteria out of the bladder and urethra

•white blood cells called neutrophils, which can find and kill bacteria

Different things can overcome and damage these natural protection mechanisms. For example:

•something may break the skin barrier, such as having a drip in your arm or a wound from surgery

•a catheter into your bladder can become a route for bacteria to get inside the bladder and cause infection

•anti acid medicines for heartburn may neutralise the stomach acid that kills bacteria

Certain cancer treatments can also overcome these protection mechanisms. Chemotherapy can temporarily reduce the number of neutrophils in the body, making it harder for you to fight infections. Radiotherapy to the lung can damage the hairs and mucus producing cells that help to remove bacteria.


These white blood cells are very important for fighting infection. They can:

•move to areas of infection in the body

•stick to the invading bacteria, viruses or fungi

•swallow up the bacteria, viruses or fungi and kill them with chemicals

Your normal neutrophil count is between 2,000 and 7,500 per cubic millimetre of blood. When you don’t have enough neutrophils in your blood, doctors may say that you are neutropaenic.

Chemotherapy, targeted cancer drugs and some radiotherapy treatments can lower the neutrophil count. So, you may be more likely to get bacterial or fungal infections after these treatments.

It is important for you to know the following when having cancer treatment:

•infections can become serious very quickly in people with low neutrophil counts

•conventional doctors give antibiotics to help prevent severe infection if your blood counts are low

You are more likely to become ill from bugs you carry around with you, then from catching someone else’s. This means that you usually don’t have to avoid contact with your family, friends or children after treatment.

You can ask your doctor or nurse what precautions you should take against infection.


Acquired immunity

This is immune protection that the body learns after having certain diseases. The body learns to recognise each different kind of bacteria, fungus or virus it meets for the first time. So, the next time the same bug invades the body, the immune system is ready for it and able to fight it off more easily. This is why you usually only get some infectious diseases, such as measles or chicken pox, once.

Vaccination works by using this type of immunity. A vaccine contains a small amount of protein from a disease. This is not harmful, but it allows the immune system to recognise the disease if it meets it again. The immune response can then stop you getting the disease.

Some vaccines use small amounts of the live bacteria or virus. These are live attenuated vaccines. It means that scientists have changed the virus or bacteria so that it stimulates the immune system to make antibodies. A live vaccine won’t cause an infection.

Other types of vaccine use killed bacteria or viruses, or parts of proteins produced by bacteria and viruses.

B cells and T cells

Lymphocytes are a type of white blood cells involved in the acquired immune response. There are 2 main types of lymphocytes:

•B cells

•T cells

The bone marrow produces all blood cells, including B and T lymphocytes. Like the other blood cells, they have to fully mature before they can help in the immune response.

B cells mature in the bone marrow. But T cells mature in the thymus gland. Once they are fully mature, the B and T cells travel to the spleen and lymph nodes ready to fight infection.

What B cells do

B cells react against invading bacteria or viruses by making proteins called antibodies. The antibody made is different for each different type of germ (bug). The antibody locks onto the surface of the invading bacteria or virus. The invader is then marked with the antibody so that the body knows it is dangerous and needs to be killed. Antibodies can also detect and kill damaged cells.

The B cells are part of the memory of the immune system. The next time the same germ tries to invade, the B cells that make the right antibody are ready for it. They are able to make their antibody very quickly.

How antibodies work

Antibodies have 2 ends. One end sticks to proteins on the outside of white blood cells. The other end sticks to the germ or damaged cell and helps to kill it. The end of the antibody that sticks to the white blood cell is always the same. Scientists call this the constant end.

The end of the antibody that recognises germs and damaged cells varies, depending on the cell it needs to recognise. So it is called the variable end. Each B cell makes antibodies with a different variable end from other B cells.

Cancer cells are not normal cells. So some antibodies with variable ends recognise cancer cells and stick to them.

What T cells do

There are different kinds of T cells called:

•helper T cells

•killer T cells

The helper T cells stimulate the B cells to make antibodies and help killer cells develop.

Killer T cells kill the body’s own cells that have been invaded by the viruses or bacteria. This prevents the germ from reproducing in the cell and then infecting other cells.

Cancer treatments that use the immune system

Some cancer treatments use elements of the immune system to help treat cancer.


Immunotherapy is a treatment for some types of cancer, for example melanoma that has spread. It uses natural body substances, or drugs made from natural body substances, to treat cancer.

They are helpful in cancer treatment because cancer cells are different from normal cells. And the immune system can recognise and kill abnormal cells.

Scientists can produce, in the laboratory, different chemicals that are part of the immune response. So, they can make different types of immunotherapy such as:

•monoclonal antibodies (MABs), which recognise and attack certain proteins on the surface of cancer cells

•vaccines to help the immune system to recognise and attack cancer

•cytokines to help to boost the immune system

•adoptive cell transfer to change the genes in a person’s white blood cells

Checkpoint inhibitors


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