7.5 “Addictive" properties of regular exercise.

A preventive medicine specialist may find the evidence for the "addictive" properties of regular exercise. The finding may also explain why athletes often fail to notice injury until after the competition is over. Dr Lee S. Berk has found that persons who exercise regularly produce high levels of a natural opiate called beta-endorphin in response to strenuous activity. This substance, a hormone produced by the brain and the pituitary gland, increase pain tolerance, counters stress, and imparts a feeling of well-being. In his study of six men and six women who were tested on a treadmill, those who jogged regularly and were physically fit, produced beta-endorphin more rapidly and in far greater amounts than those who were usually sedentary. After the activity was stopped, beta-endorphin levels dropped back to normal. In the non-runners, only a small rise in beta-endorphin occurred while they exercised. However, a larger increase in beta-endorphin production was noted some time after the activity was finished, when it was ineffective. Dr. Berk noted beta-endorphin production may also account for other benefits of vigorous exercise, such as ability to lower blood pressure and suppress appetite, both of which are known effects of the hormone. "Beta-endorphin may also explain why people become addicted to exercise", Dr. Berk said.

Did you know?

It has been estimated that the bacterium

causing tuberculosis resides in the

bodies of 2 billion people – more than

one – third of all human beings.

Summary

7.6 Types of immunity and immunization

Immunity is the ability of an organism to resist disease. It involves the recognition of foreign material and the production of chemicals which help to destroy it. These chemicals, called antibodies, are produced by lymphocytes of which there are two types: T-lymphocytes, which are formed in bone marrow but mature in the thymus gland, and B-lymphocytes, which are formed and mature in the bone marrow.

There are two basic types of immunity, passive and active.

Passive immunity is the result of antibodies passed into an individual but not being produced by the individual itself. This passive immunity may occur naturally in mammals when, for example, antibodies pass across the placenta from a mother to her fetus or are passed to the newborn baby in the mother's milk. In both cases the young developing mammal is afforded some protection from disease.

Alternatively, passive immunity may be acquired artificially by the injection of antibodies from another individual. This occurs in the treatment of tetanus and diphtheria in humans, although the antibodies are acquired from other mammals, e.g. horses. In all cases, passive immunity is only temporary.

Active immunity occurs when an organism manufactures its own antibodies. Active immunity may be the natural result of an infection. Once the body has started to manufacture antibodies in response to a disease-causing agent, it may continue to do so for a long time after, sometimes permanently. It is for this reason that most people suffer diseases such as mumps and measles only once. It is possible to induce an individual to produce antibodies even without suffering a disease. To achieve this, the appropriate antigen must be injected in some way. This is the basis of immunization (vaccination). There are a number of different types of vaccination depending on the form the antigen takes:

1. Living attenuated microorganisms - Living pathogens, which have been treated, e.g. by heating, so that they multiply but are unable to cause the symptoms of the disease. They are therefore harmless but nonetheless induce the body to produce appropriate antibodies. Living attenuated microorganisms are used to immunize against measles, tuberculosis, poliomyelitis and rubella.

2. Dead microorganisms - Pathogens are killed by some means and then injected. Although harmless they again induce the body to produce antibodies. Typhoid, cholera and whooping cough are controlled by this means.

3. Toxoids - The toxins produced by some diseases, e.g. diphtheria and tetanus, are sufficient to induce antibody production by an individual. To avoid these toxins causing the symptoms of the disease they are first detoxified in some way, e.g. by treatment with formaldehyde, and then injected.

4. Extracted antigens - The chemicals with antigenic properties may be extracted from the pathogenic organisms and injected. Influenza vaccine is produced in this way.

5. Artificial antigens - Through genetic engineering it is now possible to transfer the genes producing antigens from a pathogenic organism to a harmless one which can easily be grown in a laboratory. Mass production of the antigen is possible in a fermenter ready for separation and purification before use. Vaccines used in the treatment of hepatitis B can be produced in this way.

Supplementary reading