Sherlock MS and the Case of the Resentful Watch

Something is long over, and yet the body continues, hours, days or even weeks later, to behave as though the insult had only just been delivered. It is deeply disagreeable. And of course exactly my field. 😌

The case concerned the immune system. More precisely: its character. For decades, people had assumed that the finer sort of remembering belonged solely to the adaptive branch, that immaculately groomed specialist corps with antibodies, T cells, and the entire bureaucratic apparatus of targeted defence. The innate immune system, by contrast, was thought of rather as security staff with limited imagination: rough, quick, diligent, but hardly gifted in memory. What an aristocratic misjudgement.

For we now know that these cells, too, can remember things. Not with name tags and guest lists, naturally. Rather in the unpleasant way. They are trained. And then, the next time, they react faster, more forcefully, or at least differently, although the original cause is long gone. That, precisely, is what one calls trained immunity, a kind of built-in grudge held by the innate immune system. It arises through alterations in cellular metabolism and small lasting marks on gene regulation, so that the same cell, or its descendants, no longer begin from scratch when trouble appears again.

For lay people, I am happy to translate this into civilised language: imagine a country house. At the first burglary, the watchman is caught off guard, reaches for his whistle, trips over the runner, and takes three minutes to find the alarm bell. After that incident, the staff become nervous. Brighter lamps are installed, the key is kept elsewhere, more coffee is consumed, and suddenly, at the next suspicious noise, the entire household is in uproar far more quickly. Nobody has learnt in the manner of a barrister revising for the Bar. But the house has been primed. More irritable. More alert. A trifle resentful. ☕🚨

That is exactly how certain innate immune cells behave. They do not remember one specific pathogen with the precision of a pedantic archivist; rather, they alter their basic disposition. This is extremely useful if danger genuinely returns. It is rather less charming when the watch then begins firing at every sparrow that flies past. And here the case becomes properly delicate: trained immunity is neither saint nor villain. It is both. Sometimes it protects magnificently. Sometimes it overreacts most disagreeably and helps to drive inflammation, tissue damage, or chronic complaints. A security service with too good a memory is only a short step away from paranoia.

Now, when many people hear the phrase immune system, they immediately think of blood, lymph nodes, and some slightly exhausted GP peering at a CRP result. An understandable but crude error. For the brain, too, has staff. Microglia patrol there, one might call them the resident house detectives of the nervous system. Then there are infiltrating macrophages, reinforcements from outside, and even astrocytes, long thought to be little more than administrative personnel, though under the right circumstances they prove surprisingly opinionated. In neuroinflammatory contexts, these cells can show signs of precisely this sort of “memory”: they are shaped by earlier stimuli and react differently when later provoked,  sometimes more helpfully, sometimes more harmfully. That is what makes the matter so disagreeably fascinating.

One must not imagine the brain as some solitary queen in an ivory tower. It is more like a sensitive estate with a poor tolerance for noise. When an infection, inflammation, or other impropriety occurs elsewhere in the body, signals can penetrate all the way into the nervous system. The blood-brain barrier is not a Victorian moat with a drawbridge, but more like an exclusive door policy that, under stress, can become surprisingly permissive. And if the innate immune system has already been trained, then it is not merely personnel marching through the corridors; it is personnel marching through with opinions. 🏰

What is especially elegant about this case is that trained immunity does not simply mean “more inflammation”. Like any good scandal, it is more complicated than that. It depends on who was trained, by what, how often, and when. A short, moderate stimulus may sharpen the watch. Too much of the same stimulus may make that very same watch blunt, tolerant, or simply resigned. That happens too. The body is not a piano with a single key marked louder. It is more like an orchestra full of offended woodwind players. 🎻😏

And with that the solution lay before me like a properly laid breakfast table: trained immunity is the remarkable ability of the innate immune defence to remain altered for the long term after an initial stimulus. Not through sentimental memory, but through biological renovation. In the brain and around it, this can mean that microglia, macrophages, and even astrocytes respond differently to later stimuli, more protectively, more aggressively, more helpfully, or simply more unfortunately. In other words: the second act of an inflammatory drama is often written by the first, even if the curtain has long since fallen in between.

What I particularly admire about this case is its human quality. We all know creatures who react with rather too much vigour to old offences. Family members. Faculty committees. Occasionally violinists. In that respect, the immune system appears to be far more human than anyone had given it credit for. It does not always forget. It reshapes itself. It settles inwardly upon a position. It carries traces of the first annoyance into the second like a butler who, since Christmas 2019, still remembers who ruined the good napkin. 🫖

My brother solves cases in which someone realises too late that the culprit has returned. I solve the more elegant sort: those in which the body itself never entirely stopped remembering the first visit. 🔎🧠🕵️‍♂️

Yours, Sherlock MS