Natural defenses

The human body is constantly bombarded by an incalculable number of microorganisms, of which bacteria and viruses are the most common. Most of them are harmless, but some are agents of infection (pathogens). The latter invade the body in various ways through injuries, from infected people or animals, from contaminated food or drink, or in the air we breathe.

Pathogens have four main routes into the body: through the respiratory tract and the lungs; through breaks in the skin; through the digestive tract into the stomach and bowels; and through the reproductive and urinary tract. Once inside the body, infection may remain in the area where it arrived, or it may travel through the blood or lymphatic systems to other parts of the body.
In the face of such constant attack, the human body has a formidable array of natural defenses. It fights infections in two main ways: by preventing the entry of harmful microorganisms into the body; and by destroying or neutralizing those that do enter.

The respiratory system (above)— from the nose through to the lungs—is continually exposed to harmful agents, but has a formidable array of defenses to combat them. Hair and mucus in the nasal passages filter out dust and dirt; lymphatic tissue of the tonsils and adenoids kill bacteria and viruses; hairlike cilia (right) sweep mucus up the bronchi and trachea to the throat; and cells called macrophages (far right) engulf dust particles in the lungs.

Barrier defenses

Skin provides the main physical barrier to infection. It has a slightly acid surface that is too cold and hostile for most germs and is further protected by bactericidal chemicals in sweat.

The natural openings into the body are protected too. The sensitive mucous membranes that line the body’s orifices and internal passages are sticky so they can trap harmful invaders; they also contain bactericidal substances. The nose, for instance, filters air entering the respiratory tract. Mucus and hairs in the nostrils trap some unwanted particles; others are trapped farther along and removed by cilia of the trachea and bronchi, so that they can be expelled from the body by coughing or sneezing. Specialized dust cells in the lungs produce macrophages that engulf minute particles that pass the other defensive barriers. The acidic conditions of the stomach kill most invasive agents that are swallowed. The secretions of certain glands also serve as barriers: tears, nasal and vaginal secretions, saliva, and the digestive juices all contain antibacterial enzymes or other chemicals.

An injury to the skin results in invasion by bacteria, which are met by the body’s second line of natural defenses, the blood. Phagocytes migrate to the injured area and engulf invading bacteria. Blood platelets, which have an important role in blood clotting, also concentrate at the site of bleeding.
Blood clotting helps to seal the wound so that tissue repair can proceed. Phagocytes continue to destroy bacteria, while spent phagocytes and dead bacteria accumulate as pus.
New tissue repairs the damage at the site of the wound. Antibodies formed in response to the invading antigens help to eliminate any remaining bacteria and confer a degree of lasting immunity.
Scarring can be extensive at the site of a large and ragged wound. This photograph shows a wound that was stitched to close it and minimize scarring, but nevertheless the skin has been distorted by inelastic scar tissue.

Internal defense systems

Where pathogenic organisms do manage to gain access to the body’s tissues, complex internal defenses take over, responding directly to the physical presence of invading organisms and to the toxins produced by them.

Phagocytes are specialized cells that engulf and digest harmful bacteria. Some of these are highly mobile, such as the white blood cells called polymorphonuclear leukocytes or the single-nuclear monocytes that circulate in the

blood. When a cut or abrasion results in local bacterial invasion, increased numbers of phagocytes migrate to the invaded area, where they surround and ingest the bacteria to prevent them from spreading.

Other phagocytes, called macrophages, are more permanently located in specific parts of the body particularly the lungs, spleen, liver, and lymph nodes where they filter infective agents out of the blood and lymph circulation.

The lymphatic system plays a crucial role in the body’s “immune response,” the highly complex defense mechanism by which antibodies are formed in direct response to the presence of antigens foreign substances, usually proteins, produced by invading organisms. Special white blood cells called lymphocytes, which are formed in the lymphoid tissue of the bone marrow, thymus, lymph nodes, spleen, tonsils, and adenoids, detect any alien protein that enters the bloodstream.

Lymphocytes and plasma cells then produce a protein called an antibody. Released into the bloodstream, the antibodies attack the antigens. Some cause bacteria to clump together, and so prevent their spread; others affect the surface of bacteria and make them vulnerable to phagocytes; still others (antitoxins) neutralize poisonous toxins produced by antigens.

Antibodies are not general in their action. Those produced in response to the antigens of diphtheria bacteria are not able to react with those of tuberculosis bacteria, for instance. Nor does the body produce large numbers of antibodies on immediate exposure to an antigen. Large-scale production of antibodies occurs only a few days after exposure; it recurs much more rapidly during subsequent exposure. Once antibodies have been formed in response to a particular bacterium or virus, however, they never completely disappear from the bloodstream and so form the basis of lifelong immunity against that particular illness. The best-known example is the immunity acquired to childhood diseases such as measles. Following the same principle, vaccines containing dead or weakened pathogens confer artificial immunity by stimulating antibody production.

Pathogenic microorganisms are not the only foreign proteins that bring about antibody production. The body’s tendency to reject skin grafts or transplanted organs is also due to its built-in ability to recognize foreign tissue.


Infection usually results in cellular damage. Where this occurs, special mobile cells called fibroblasts lay down strands of fiber to form a scaffolding onto which new cells can grow from healthy, neighboring tissue. In open wounds, the ability of blood to clot is important in preventing excessive loss of blood. The process consists of a complex series of chemical reactions but culminates in the conversion of a fluid substance, fibrinogen, into a mass of thin protein threads (fibrin), which forms the core of the clot.