The Human Immune System

We begin, then, with the immune system. Nature is wonderful - it is truly beautiful - and it is very nice indeed to wax lyrical about it. I, however, am not very good at that and, as this is my work, I'm going to play to my strengths. Nature is full of organisms which exist to reproduce. Life, to borrow slightly from the words of Kahlil Gibran, longs for itself - living things exist to make more of themselves. In order to do this, they need to survive - at least for as long as it takes to successfully reproduce.

Surviving generally requires organisms to make use of other organisms; unfortunately, many organisms do this in destructive ways - the lion preys upon the giraffe, the giraffe feeds on the acacia tree, the Salmonella bacteria infect the human. Not all such uses are destructive, of course - for instance, many plants have a symbiotic relationship with certain animals, in as much as the seeds of those plants are effectively dispersed by the consumption of the fruit. However, many living things are in the business of attacking and harming other living things; perhaps for food, perhaps for a host.

In the spirit of reproduction, defences have evolved against attacks by other organisms. Some of these defences are impressive, dangerous things - like horns. One defence, however, is genuinely much more wondrous. To protect organisms from infection and disease, immune systems have evolved. Some are fairly simple, as in the case of bacteria. The human immune system, however, is a magnificent barrier to infection, disease and - ultimately - death.

As well as protecting organisms against attacks by other living things - such as bacteria - the immune system, of course, protects against pathogens generally, as well. I stress this point because there is debate over whether viruses constitute a form of life. In short, however, the immune system exists to protect living things from infection and disease - caused by infectious agents, many of which are other living things. This is, ultimately, so that living things can reproduce.

One of the most important barriers to infection and the "first defence" - if you like - is the skin. The skin is of vital importance as a mechanical barrier - it serves as something of a wall, physically preventing pathogens - as well as dirt, toxins, poisons and other such undesirable things - from entering the body. This is one reason why severe burns pose such a serious threat. Assuming a patient survives their initial injuries, the loss of skin leaves patients extremely vulnerable to infection, which can cause complications and lead to the death of the patient, who may have been recovering from the burns themselves.

The skin is not the only barrier, however. Complex organisms such as human beings cannot go around sealed in their skin, they need to take materials in and they need to send materials out. Breathing and eating, for example, can potentially significantly compromise the body's defence, which is a little unfortunate. As a result, we have more barriers. The respiratory tract is very sensitive and the reflexes of coughing and sneezing can effectively expel any pathogens that are detected from the body. Unfortunately, as regular travellers on public transport will be aware, this has the effect of spraying them about the place. Furthermore, mucus acts to engulf and trap pathogens - which are then expelled with the mucus - and urine and tears can also flush pathogens from the body. The body also produces a number of antimicrobials and antibacterials. Both the skin and the respiratory tract produce antimicrobial peptides, e.g. β-defensins. Saliva, tears and breast milk contain antibacterials, e.g. the enzyme lysozyme. The human vagina is slightly acidic and the fluids which make it so help to protect against pathogens. Stomach acid, too, destroys many pathogens and - alongside its primary function of digestion - has an important role to play in mitigating the compromising effect of our need to ingest food. It is interesting to note that the presence of flora in the genitourinary and gastrointestinal tracts serves as a barrier against infection as well, as these bacteria provide competition for resources.

Naturally, however, these extensive defences are often breached - partly because humans have a disobliging habit of regularly breathing, eating, copulating and the like. Should pathogens penetrate these defences, other, more destructive elements of the immune system take over. We will look at these in great detail in this chapter; but, before we do so, it is important to distinguish between two categories of immunity:

• Innate immunity. Innate immunity is nonspecific and it is innate - a person is born with it. It is an (evolutionarily) much older, much simpler form of immunity and is also found in plants and simple multicellular organisms.


• Adaptive immunity. Adaptive immunity is much more sophisticated. Developed over time, adaptive immunity confers immunity to specific pathogens and allows for specific responses to specific infections. Furthermore, immunological memory allows for the future recognition of pathogens which have been encountered before, allowing for a faster, more effective response.