I wanted to write something in time for World Homeopathy Awareness Week since homeopathy is such an amazing phenomenon. But though the promotional week is now over, I’m sure you can remember it. You will have memories of some of the things that were written and said.

Memories are so important for human beings. They enrich the present and allow us to navigate from the past into the future. We like to wallow in the pleasant ones, ever returning from holidays — as my family did from Venice last weekend — laden with photographs and souvenirs to enhance our store of recollection. Even bad memories are, like Venetian souvenir shops, almost impossible to avoid, such is their ubiquity in our experience.

Memory is also central to the modern interpretation of homeopathy, the complementary medical practice of treating like with like in which substances that induce certain disease symptoms are serially diluted in water and succussed (i.e. shaken) to be ‘potentised’ as remedies. The technique was first proposed at the end of the 18th Century, prior to the development of our present-day theories of the atomic and molecular nature of matter. These revealed homeopathic dilutions to be so extreme that not a single molecule of the original symptom-inducing substance remains in the preparations used for treatment. To explain the efficacy of the remedies, it has now been advanced that water retains a memory of the molecules that have been diluted away and thereby transmits their therapeutic benefit.

As I said, it’s an amazing phenomenon.

Curiously, homeopathy appears to echo the body’s natural healing process, which also relies on memory to combat disease and illness. The adaptive immune system is a thing of wondrous complexity but I just want to tell you about one small part of it to reveal something of how memory works that can also shed light on homeopathic mechanisms.

The immune system battles day by day, hour by hour, minute by minute against an onslaught of mostly microscopic invaders. A key element of this complex defence mechanism is the population of B-cells, which are producers of molecules known as antibodies.

The human body has the capacity to generate enormous numbers of different antibody molecules: the DNA that codes for antibodies within each B-cell is split into pieces and only assembles into its final form — from a large choice of similar but not identical parts — as the cell matures in the bone marrow. In a sense every B-cell contains a deck of DNA cards for antibody parts but can only use the hand that it is dealt during maturation to make one type of antibody molecule. When it is released into the circulation, each new B-cell therefore carries and codes for a unique antibody. But there are billions and billions of B-cells and the quasi-random assemblage of parts means that a huge diversity of different antibody molecules is made.

The antibody — initially displayed on the surface of the B-cell — is Y-shaped, with two identical arms. The end of each arm, (the most variable region between different antibodies — indicated by arrows in the picture here) can stick to the surface of an invading pathogen, but will only do so if it happens to make a good fit with the molecule displayed on the surface of a microbial pathogen. The body relies entirely on its capacity to generate many different antibodies to make sure there is at least one that will stick to the invader.

The picture below shows in spectacular detail the intimate interaction between a haemagglutinin molecule from the surface of a ‘flu virus that has just been found by an antibody that can latch on to it.

Can you see how closely the shape of the antibody fits the contours of the haemagglutinin molecule? The embrace is intimate but rigid. Solid. Though there may be some flexing as the two molecules come together, the association is tense one, more like wrestlers locked in a stalemate than lovers sleepily caressing in the early morning.

I have deliberately revealed the underlying structure of bonded atoms (red oxygens, blue nitrogens and cyan or yellow carbons) in the antibody molecule on the right of the picture. If you think the molecular structure is horrendously complex, then good: that’s because it is. The antibody is elaborate and convoluted because it has to be complementary in shape and surface chemistry to the structure of the protein on the microbial invader that it targets. Its conformation is fixed by the sequence of amino acids making up the protein chains of the antibody, which dictate precisely how it folds into a unique structure that happens to bind — in this case — to the surface of haemagglutinin.

We call this binding ‘recognition’ though it is recognition by touch rather than sight and — at least at first — occurs only by chance. It is not based on memory, but crucially allows a memory to form.

The initial contact between the antibody on the B-cell surface with the ‘flu protein causes the B-cell to divide, producing many daughter cells that release free antibody molecules of exactly the same structure into the bloodstream. These rove around and tag for destruction any other ‘flu viruses in the neighbourhood. In this way — though I am simplifying greatly — the infection is eventually mopped up.

But just as importantly, some of the B-cells turn into long-lived memory cells that retain the ability to make the antibody that ‘recognised’ the ‘flu haemagglutinin. Their function is to be on the look-out in the weeks, months and years to come for re-infection by the ‘flu virus. If this happens, an encounter between the antibody on the memory cell surface and the ‘flu protein is sufficient to trigger a very rapid response, which again floods the circulation with antibody molecules of the same structure, thereby preventing the infection from getting established.

Working together, B-cells and antibodies can lay down powerful immunological memories. The secret of the success of this system is in the preserved structure of the molecules, encoded by the DNA sequence of the shuffled antibody gene and expressed in the ornate three-dimensional architecture of the protein chains that form the antibody.

With World Homeopathy Awareness Week fresh in our minds, it is appropriate to ask: why did evolution, which has always taken such a make-do attitude to developing the faculties of living things, not choose to work with water to construct an immune system capable of remembering its enemies? Surely, given the prodigious feats of memory attributed by homeopaths to this glistening, life-giving fluid, Nature could have fashioned a watery armour to protect us from disease?

This has not happened of course because memories require fixed structures and water is a liquid. In this form, it is no more capable of remembering molecules than of being sculpted.

Instead the immune system revealed by scientific analysis shows us that the sophisticated mechanism needed to fight off infection has been built from shaped materials. The potency of an antibody is derived from its ability to form a structure that is uniquely specified by the sequence of amino acids in the protein chain. The potency of the B-cells that give rise to them lies in the eloquent DNA structure through which the amino acid sequence is coded. And remembered.

The antibody molecule in the picture above may be off-putting to some because of its near impenetrable irregularity. But to my mind — and I confess to the predilections of a structural biologist here — it represents a winsome confluence of form and function. Paradoxically perhaps, the elegance of that understanding was won by brutish methods. Time and again we have found that only by asking the harshest questions and sifting our data with the severest eye can we lay a hand on those bright nuggets of insight that are worth keeping.

If you take a softer, gentler approach that is satisfied by more superficial observations, then understanding just leaks through your hands, like water.