Immune System
Every day of our lives we come into contact with a large variety of microorganisms such as viruses, bacteria and fungi. Many of these, known as pathogens, are capable of causing disease, yet most of the time we do not succumb to infection. Moreover when we do, the course of disease is usually short-lived. The reason we are able to ward off these microbes is because of our natural defenses which are known as the immune system. This system manages to eliminate very efficiently most incoming microorganisms, often without us realizing we have even been infected.
Its importance can best be seen in those patients in whom it is defective. This situation, known as immunodeficiency, results in frequent, overwhelming and sometimes life-threatening infections. Perhaps the best known cause of such a condition is infection with the virus HIV leading to Acquired Immunodeficiency Syndrome or AIDS. This virus infects and destroys particular cells of the immune system, without which the patient becomes very susceptible to a variety of bacterial, viral and fungal infections. It is these secondary infections and not the original HIV infection which are life-threatening. Immunodeficiencies are also caused by certain rare genetic diseases. Perhaps the best known of these is Severe Combined Immunodeficiency which made the headlines in the 1970's with the famous case of David, "the boy in the bubble". Infants born with Severe Combined Immunodeficiency usually die of overwhelming infections within the first year or two of life. However David was placed in a large plastic chamber which was kept completely sterile. He survived in this chamber until he was twelve, though eventually he too died of infection.
Another common immunodeficiency occurs following treatment with drugs which damage the immune system. For example, immunodeficiency is a common side effect of chemotherapy for cancer and patients undergoing such treatment become more susceptible to infections. All of these examples illustrate how we are being continuously exposed to potentially dangerous microorganisms and how most of the time our immune system eliminates them without us even being aware of the threat.
So, how does the immune system provide this kind of protection? The study of the immune system is relatively recent and its origins can be traced to Edward Jenner in the late eighteenth century. He discovered that humans could be protected against smallpox by inoculation with cowpox, also known as vaccinia, a relative of the smallpox virus. This procedure became known as vaccination and has become one of the most effective methods of combating infectious diseases. The principle of vaccination is that the vaccinated individual develops a specific immune response against cowpox which, because of the similarity of the viruses, also provides protection against smallpox. So effective is this procedure that a World Health Organization driven vaccination campaign against smallpox finally eliminated the disease completely in 1979, the first, and so far only, human disease to have been completely eradicated.
The next major advance in the study of the immune system came at the end of the nineteenth century with the discovery that the blood serum of animals that had been vaccinated contained substances known as antibodies, which specifically bound to the pathogenic organism. These, if transferred experimentally to another animal, resulted in the recipient becoming protected against the infection. Antibodies turned out to be proteins known as immunoglobulins which circulate in the blood and are a crucial component of the immune system's defense against pathogens. The substances to which antibodies bind, for example viral proteins, are called antigens and it was shown that animals can make antibodies specific for a vast array of different antigens. How this is accomplished became a major focus of research for many decades. Work at the National Institute for Medical Research and at the Rockefeller Institute in New York unravelled the structure of immunoglobulins and showed that the portions of these molecules capable of binding to antigens were extremely variable. Further work showed that the way this variability was achieved was by an unusual mechanism operating at the level of the genes which contain the information required to make immunoglobulin proteins. It turned out that these genes exist in hundreds of small segments which are joined together in many different combinations to produce immunoglobulin genes with sections that are extremely variable, resulting in the potential production of tens of millions of different immunoglobulins. This vast diversity is the key to the ability of organisms to mount effective immune responses against the huge array of different pathogens they may encounter.
Every day of our lives we come into contact with a large variety of microorganisms such as viruses, bacteria and fungi. Many of these, known as pathogens, are capable of causing disease, yet most of the time we do not succumb to infection. Moreover when we do, the course of disease is usually short-lived. The reason we are able to ward off these microbes is because of our natural defenses which are known as the immune system. This system manages to eliminate very efficiently most incoming microorganisms, often without us realizing we have even been infected.
Its importance can best be seen in those patients in whom it is defective. This situation, known as immunodeficiency, results in frequent, overwhelming and sometimes life-threatening infections. Perhaps the best known cause of such a condition is infection with the virus HIV leading to Acquired Immunodeficiency Syndrome or AIDS. This virus infects and destroys particular cells of the immune system, without which the patient becomes very susceptible to a variety of bacterial, viral and fungal infections. It is these secondary infections and not the original HIV infection which are life-threatening. Immunodeficiencies are also caused by certain rare genetic diseases. Perhaps the best known of these is Severe Combined Immunodeficiency which made the headlines in the 1970's with the famous case of David, "the boy in the bubble". Infants born with Severe Combined Immunodeficiency usually die of overwhelming infections within the first year or two of life. However David was placed in a large plastic chamber which was kept completely sterile. He survived in this chamber until he was twelve, though eventually he too died of infection.
Another common immunodeficiency occurs following treatment with drugs which damage the immune system. For example, immunodeficiency is a common side effect of chemotherapy for cancer and patients undergoing such treatment become more susceptible to infections. All of these examples illustrate how we are being continuously exposed to potentially dangerous microorganisms and how most of the time our immune system eliminates them without us even being aware of the threat.
So, how does the immune system provide this kind of protection? The study of the immune system is relatively recent and its origins can be traced to Edward Jenner in the late eighteenth century. He discovered that humans could be protected against smallpox by inoculation with cowpox, also known as vaccinia, a relative of the smallpox virus. This procedure became known as vaccination and has become one of the most effective methods of combating infectious diseases. The principle of vaccination is that the vaccinated individual develops a specific immune response against cowpox which, because of the similarity of the viruses, also provides protection against smallpox. So effective is this procedure that a World Health Organization driven vaccination campaign against smallpox finally eliminated the disease completely in 1979, the first, and so far only, human disease to have been completely eradicated.
The next major advance in the study of the immune system came at the end of the nineteenth century with the discovery that the blood serum of animals that had been vaccinated contained substances known as antibodies, which specifically bound to the pathogenic organism. These, if transferred experimentally to another animal, resulted in the recipient becoming protected against the infection. Antibodies turned out to be proteins known as immunoglobulins which circulate in the blood and are a crucial component of the immune system's defense against pathogens. The substances to which antibodies bind, for example viral proteins, are called antigens and it was shown that animals can make antibodies specific for a vast array of different antigens. How this is accomplished became a major focus of research for many decades. Work at the National Institute for Medical Research and at the Rockefeller Institute in New York unravelled the structure of immunoglobulins and showed that the portions of these molecules capable of binding to antigens were extremely variable. Further work showed that the way this variability was achieved was by an unusual mechanism operating at the level of the genes which contain the information required to make immunoglobulin proteins. It turned out that these genes exist in hundreds of small segments which are joined together in many different combinations to produce immunoglobulin genes with sections that are extremely variable, resulting in the potential production of tens of millions of different immunoglobulins. This vast diversity is the key to the ability of organisms to mount effective immune responses against the huge array of different pathogens they may encounter.
