The human body contains a dazzling collection of inter-related systems. These systems require exquisite balance and constant maintenance to ensure correct function and a healthy body. Unfortunately the body also moves through a world filled with potential dangers that can undermine this balancing act. Bacteria and viruses can invade, disrupting the functions of organs and cells, and sometimes even turn the body upon itself. Physical and chemical trauma can destroy tissue and open the way for these pathogens to enter the body. Fortunately the body also has a system to defend itself from invasion and damage. This is the immune system.
The environment around us is inhabited by an enormous number of microorganisms and as a result the human body is colonised from the moment of birth. At least 500 different species of bacteria naturally live on or in the human body with approximately 400 in the gastrointestinal tract. Of all the cells in a human body the overwhelming majority are non-human, outnumbering human cells by 10 to 1 (100 trillion bacterial cells versus 10 trillion human cells). Amongst these microbes are a variety of bacteria that take up almost permanent residence within the body. These are known as indigenous bacteria or normal flora. These microbes are usually symbiotic with the host and provide a number of beneficial services.
This normal flora of the body is a complex ecosystem, which is regulated by the diet, microbial interactions and host factors such as the motility of the gut and intestinal secretions. External factors such as stress, dietary changes and medications can affect the normal flora and alter the types of organisms present or their metabolism. If the balance is disrupted it can be detrimental to the host and can cause disease.
As mentioned above, the human body is a complex system of checks and balances that moves through a world replete with potentially harmful factors. Fortunately the body’s physical and chemical barriers to entry and internal defensive processes are often effective at keeping invading organisms at bay. Sometimes however the balance is lost and invading pathogens are successful in penetrating the body’s defences causing infections or triggering inappropriate immune responses. Similarly the balance between host and indigenous flora can be compromised leading to infection by normally avirulent organisms, e.g. episodes of thrush (Candida albicans infection) following the use of antibiotics.
One of the most famous examples of infectious disease involving the lymphatic, cardiovascular and respiratory systems is the Black Death (pneumonic and bubonic plague). Spread by fleas carried on rats, the Black Death killed 25% of the population of Europe (approximately 20 million people) and approximately half the population of England in the period 1347 to 1350. The plague is caused by a bacterium Yersinia pestis which when untreated is fatal in 50 to 75% of bubonic plague sufferers and nearly 100% of pneumonic plague cases. Today the use of antibiotics such as streptomycin and tetracyclines has reduced the fatality rate in the few cases that occur in developed countries to 1 to 5%.
Although a variety of natural mechanisms are available in the human body, they are not specifically directed at a given pathogen. This can lead to the situation of too little, too late. Consequently these mechanisms are often ineffective in dealing with attack by microbes and their toxins.
Adaptive immunity, as the name suggests, changes after initial exposure to a pathogen. The body reacts faster and stronger the second time around, compared to the first challenge, as can be seen in Figure X. This magnified reaction comes as a result of the ability of the immune system to remember a given pathogen and rapidly respond to it specifically, i.e. the development of immunological memory.
Vaccines are based on this concept of immunological memory. The name vaccine (vacca = cow) was coined after Edward Jenner in the 1700s inoculated a small boy with cowpox and observed that the boy was then protected against subsequent exposure to smallpox. The essential strategy for vaccines is to prepare an innocuous form of the infectious organism or its toxins. This form still substantially retains the chemical structures responsible for producing the immune response (antigens), without those elements responsible for its pathogenic activity. The body will then produce a vigorous and highly specific response to the antigens should the antigen present again in the future.
It has been estimated that by the time a child reaches 18 months old, over 25 needle sticks for vaccination have been administered. This figure leads to a considerable amount of distress for the child and its parents, with resulting poor compliance and loss of effective immunisation. In response to this many polyvalent vaccines are in development, but these have the disadvantage of costs – a factor that takes effective vaccination out of the reach of many disadvantaged families – and concerns over the impact of multiple delivery of antigens, for example in the case of the measles, mumps and rubella (MMR) vaccine