More information about Immunity and Infection

Many other chemicals play a part in the immune response including the interferons, whose secretion is stimulated amongst other things by viral infection, complement, which is a collection of nine different chemicals which combine in various ways to attack pathogens, and substances called interleukins. There are other cells besides the neutrophils and the lymphocytes which are also involved in fighting infection, such as macrophages (which devour bacteria and dead cells) and mast cells that contain, among other things, histamine, which is released under the influence of antibodies and which invokes part of the inflammatory response necessary to overcome the invading pathogen.

We can become infected in various ways: we can inhale germs, we can swallow them on contaminated food, we can contract them through injury and wounds that break the skin, through blood transfusion or through sexual contact. Many microbial infections can be treated using antibiotics - viral infections cannot. Infection causes fever: our body temperature is controlled by a part of the brain called the hypothalamus, and is set at 37 degrees Celsius. Infection by a bacterium or virus causes this thermostat to be reset at a higher level, and our temperature rises, producing fever. This is accompanied by shivering (the repeated muscle contractions contribute to the rise in temperature) headache (because the blood vessels are dilated) and sweating as the body tries to lose heat. A temperature above 40 degrees can produce delirium. A raised temperature can be reduced using agents that reset the thermostat (such as paracetamol or aspirin) and by tepid sponging and fanning to increase heat loss from the body surface.

Immunity

Can be boosted by the process of vaccination, in which the body is exposed to a harmless variant of the infectious agent so that the immune system learns to recognise it - and is therefore ready and waiting when the real agent arrives. Vaccination has reduced the incidence of many diseases worldwide, and has resulted in the elimination of smallpox. Some vaccines, such as that against polio, are live viruses which have been weakened, or attenuated. Others, such as those used against influenza, are made up of pieces of dead viruses. There are, unfortunately, many diseases for which vaccines are not yet available. Vaccination produces active immunity - our immune system is activated. For some diseases immunity can be improved by the use of injections of gammaglobulin extracted from the blood of people with the disease. This is passive immunity, because the gammaglobulin gets degraded over a period of time, and the immune system is not activated.

Viruses

Are very small particles which consist of an inner core of genetic material (either DNA or RNA) covered by a coat made of protein, sometimes surrounded by a layer of lipids. Viruses are fairly primitive in terms of development, and do not contain the machinery necessary to reproduce themselves, so in order to do this they have to invade living cells and borrow their machinery. Having done so the cell becomes packed with viral particles which then burst out, destroying the cell, and spreading to infect more cells and reproduce further. Because replication of viruses occurs within our own cells they are difficult to treat, although there are now some anti-viral agents which are partially effective. Antibiotics, which are directed against bacteria (described in the section on bacterial infection) are ineffective against viruses. We overcome viral infection as a result of our own immunity - the white blood cells called lymphocytes either attack the viruses directly or secrete antibodies which destroy them. We can prime our immune system by using vaccination, whereby a killed or weakened version of the virus is introduced in order that the immune system learns to recognise it - a process which has effectively eradicated smallpox from the world, but which is unfortunately not available for many viruses, including those that cause the common cold, as well as those that cause Aids through H I V. Viruses are spread by different routes: those that are spread by droplet infection (whereby we inhale infected saliva spread by coughing sneezing or kissing) may be difficult to avoid during epidemics. Those that are the spread by the faecal - oral route (they replicate in the gut and are excreted in the faeces, and are then reintroduced to another person either by eating shellfish caught in water contaminated by sewage or as a result of poor hygiene) can be reduced by avoiding contaminated water and improving hygiene. Those that are spread by the so-called parental route, which means via blood products or sexual contact, can be avoided by screening blood products before transfusion, and practising safe sex.

Bacteria

Are microbes which consist of a single cell with a nucleus that contains only one chromosome and does not have a nuclear membrane, a family of organisms called prokaryotic. They are regarded as being more primitive than plant or animal cells, which have a nucleus containing more than one chromosome surrounded by a nuclear membrane, and so-called eukaryotic. There are a number of different families of bacteria, named according to their shape: round ones are called cocci, Rod shaped ones are called bacilli, comma- shaped ones are called vibriae, spirochaete is the name given to those shaped like a corkscrew, while the spirilla are, as would be expected, spiral shaped. All bacteria have a rigid cell wall, some of them also have an outer capsule. The structure of the cell wall determines whether or not the bacterium can be dyed using a particular stain called a Gram stain - used by microbiologists. If it can then it's called Gram positive, and if it can't then it’s called Gram negative. By using this and other techniques people who study microbes, called microbiologists, can identify bacteria in samples taken from sites of infection.

Not all bacteria are harmful to us: we depend on those that occupy the gut to break down food and waste products (the bacteria in the gut are called bacterial flora), and our skin is covered, or colonised, by a number of different bacteria which are known as commensals. Many bacteria are, however, harmful - or pathogenic (because they cause pathology) - such as the streptococcus, which causes septicaemia and pneumonia, the spirochaete Treponema palidum which causes syphilis, or the bacilli of the family Clostridium (one of a group of bacteria called anaerobic, because they exist and grow in the absence of oxygen) which cause diseases such as gangrene or tetanus. The bacteria have different methods of causing disorder; they can do so by invading tissues and invoking the inflammatory response, or they can do so by secreting a toxin as, for example, in toxic shock syndrome.

Antibiotics

Were originally defined as substances produced by one microorganism which are toxic to others - the classic example being penicillin, which was grown from a mould and is effective against many streptococci, but the word is also used to describe new agents which are synthesised in laboratories. There are many different antibiotics which utilise the fact that there are differences between the bacterial cell and our own cells - human cells don't, for example, have rigid cell walls, so many agents including the penicillins interfere with the formation of bacterial cell walls; other antibiotics work in different ways. One problem, however, with the increasing use of antibiotics is the development of resistance, because the antibiotics kill off all the susceptible bacteria, leaving more space for the resistant ones to grow. Unnecessary use of antibiotics (often prescribed in response to pressure from patients who wrongly believe that there is a pill for every ill, and are reluctant to accept that antibiotics are ineffective against viral infections, by far the most common sort of infection that we suffer) is responsible for the emergence of multiply resistant bacteria such as MRSA, described in this section.

Travel to exotic parts of the world has become increasingly common over the past decade, with more of us being exposed to illnesses which do not exist in our own countries. Many infectious agents only survive in the tropical areas of the world where the heat and humidity allow them to thrive - the agent that causes malaria, described below, being an example - but once infected they find us hot and humid enough to continue to cause disease when we return home. Many tropical diseases can be prevented by vaccination or prophylactic treatment - or commonsense measures to avoid infection. Before travelling, take travel advice from a travel clinic. Many vaccinations take time to become effective - so seek advice long before travelling.