A primer in Immunology

Immunology is generally not a student favourite. This is in part because it is probably the most acronym ridden subject in the already acronym overloaded biomedical scientific world, and it has the most obscure new vocabulary to learn. That said the immune system itself is remarkable, and it is a shame that revealing its wonders to the world is frequently so painfully done. It is a subject for another day that University staff are recruited mainly for the research glamour (and cash) they bring to their Departments and that many seem to have little or no formal training in teaching. Do ask the next Medical Student you meet if you doubt this.

Our immune system protects us from the many microbes that wish to infect us. There are around 1,400 that have so far been documented as causing infections in humans. It also has some protective benefit against cancers but this is a spin-off since it didn’t evolve for that purpose.

Broadly our immune is divided into two systems, the innate and the adaptive.

The innate immune system is what keeps us all infection free day-to-day and provides a hugely effective barrier to microbes crossing the skin or gut to get into our blood or tissues. It is a diverse ‘system’ including antiseptic substances in tears, the fact we shed skin scales (and their adherent bugs), the cough reflex; we have protective proteins on our skin and in our blood and we also have cells, phagocytes that eat invading microbes. External damage, heat, trauma etc. triggers an ‘inflammatory’ response in the innate immune system with the well-known features of redness, heat, swelling and pain (rubor, tumor, calor and dolor as described by Celsus). Think of sunburn and you get the basic idea. This response helps amplify the ability of the innate system to clear infection, should it be part of the damage. The innate immune system is remarkably powerful and it is all there is to protect animals other than vertebrates. It is undoubtedly the reason why it is so hard to catch HIV, at least by sexual contact, as the innate defences stop the virus getting in. Its limitation is that it does not change. The response is exactly the same the first and the thousandth time that you meet a damaging stimulus or a dangerous bug.

The adaptive immune system is the icing on the cake for vertebrates and is at its most sophisticated in humans. It is without doubt one reason, along with our complex brains and opposable thumbs, for our success as a species. The adaptive system is just that. Once it has encountered a bug it remembers it and next time the same one arrives, the antibodies and the killer cells which were selected as being the best to defend us when they first met the danger are already primed. They respond faster the second (and subsequent) times and get better and better at their defensive role.

In carrying out their attacks on the repeated attempts at invasion by microbes they recruit some of the innate system too including some of the proteins and hormones that activate defences more generally. It is these, substances such as interferon, that make us feel ill when we have infections like ‘flu. Much of the adaptive immune response takes place in lymph nodes, often inaccurately termed ‘glands’ which occur at junctional bottlenecks such as where the arms and legs join the body, the armpit and groin. If there is an immune response locally they can get swollen which stretches their outer capsule causing discomfort.

Thus it is today after my third rabies vaccination yesterday that my innate system set up the same painful response at the site of injection that it did the first two times. The injection site swelled up and it is now going down.
My adaptive immune system however was, after two previous encounters, on a hair trigger. In the words of Goldfinger to James Bond, my adaptive response has said this is not a first encounter (happenstance), nor is it coincidence (a second encounter) but clearly enemy action (a third). This is perhaps a long-winded and tortuous way of leading up to the fact that today I feel generally unwell and have a swollen painful lump under my arm. I can tell you why though and perhaps it explains to those to whom immunology is a dark art why the same happens to them. There is a mysterious reassurance to knowing what is going on and having a label for it – a diagnosis – something that people often seem to crave more than a cure.

At £42 per shot the rabies vaccine is not cheap, but it is paradoxically encouraging that I feel like this because £126 is peanuts if my adaptive immune system responds to the usually fatal rabies virus this violently – should they ever meet.

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