Function: The Body’s Defences

Several barriers and responses work together to protect the body against infection and the development of cancer. Physical and chemical barriers and the inflammatory response form the first two lines of defence. If invading organisms break through these general lines of defence, the body’s immune system fights back with two extremely effective immune responses that are specific to different invaders.

Physical and chemical barriers

The skin and the mucous membranes, which line the body openings and the internal passages, are effective barriers against invading organisms. Saliva, tears, mucus, sebum, sweat, and acid aid these barriers in protecting the different parts of the body.


Tears from glands in the eyelids wash away dirt and contain an antiseptic substance.


Glands in the stomach lining produce hydrochloric acid, a powerful agent that kills most invading organisms.


The skin is protected by an oily substance, sebum, produced by sebaceous glands and by sweat. Both of these are mildly antiseptic.

Genital and urinary tracts

Harmless bacteria within the genital tract and the flushing action of urine in the urinary tract both help to prevent the growth of harmful organisms.


Goblet cells in the lining of the intestines produce mucus, which protects the lining from digestive chemicals and harmful organisms.

Respiratory tract

The lining of the respiratory tract produces mucus, which traps organisms. Tiny hairs (cilia) move mucus to the throat.

The inflammatory response

If foreign organisms such as bacteria overcome the body’s physical and chemical barriers, the next line of defence is the inflammatory response, characterized by redness, pain, heat, and swelling at the damaged site.

Foreign organisms invade the body through skin that has been broken due to injury. Instantly, the damaged tissue releases specific chemicals that attract specialized white blood cells called phagocytes.

The chemicals cause the underlying blood vessel to widen and the flow of blood to increase, leading to symptoms of inflammation. The vessel walls become slightly porous, allowing phagocytes to reach, engulf, and destroy the foreign organisms.

Destroying foreign organisms

Here, a white blood cell called a phagocyte has started to engulf a foreign organism. To destroy the organism, the phagocyte releases enzymes that help to break it down.

The antibody immune response

This specific immune response targets invading bacteria and relies on white blood cells called B-lymphocytes or B-cells. These cells recognize proteins (antigens) of invading bacteria and multiply to produce antibodies. The antibodies seek out the bacteria and lock on to them, causing their destruction.

The bacteria enter the body. Some of them are engulfed by cells called phagocytes that bring the bacterial antigens into contact with B-cells, some of which match the antigens.

The matching B-cell multiplies to produce two types of cell: plasma cells, which produce antibodies to destroy the bacteria, and memory B-cells, which are stored in the body for future use.

The antibodies that are released by the plasma cells seek out and lock on to bacterial antigens and inactivate the bacteria. The antibodies also attract more phagocytes to the site to destroy the bacteria.

The cellular immune response

This type of specific immune response targets viruses, parasites, and cancer cells. It depends on white blood cells called T-lymphocytes or T-cells. After recognizing a foreign protein (antigen), T-cells multiply and engage in a direct battle against infected cells or cancer cells.

Cells that are infected by the invading viruses are engulfed by cells called phagocytes. These bring the viral antigens into contact with T-cells, some of which match up with these antigens.

The matching T-cell multiplies in order to produce different types of cells, including killer T-cells, which contain toxic proteins, and memory T-cells, stored to protect the body against the virus in the future.

Killer T-cells lock on to the infected cell bearing the recognized antigen and release their toxic proteins. These proteins then destroy the infected cell. Killer T-cells may then go on to seek out other infected cells.

Allergic reactions

An allergy is an inappropriate immune response to a normally harmless substance, called an allergen. On initial exposure to the allergen, the immune system becomes sensitized to it. During subsequent exposures, an allergic reaction occurs. Mast cells, located in the skin, nasal lining, and other tissues, are destroyed, releasing a substance called histamine that causes an inflammatory response, irritating body tissues and producing allergy symptoms.

On repeat exposure to an allergen, antibodies previously produced in response to it bind to the surface of mast cells, which contain histamine.

The allergens bind to and link two or more antibodies, causing the cell to burst and release the histamine within. Histamine causes the symptoms of allergy.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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