2025 Salam’s Distinguished Lectures

Aleksandra Walczak is a research director in biophysics at the Ecole Normale Superieure in Paris, where she co-leads the statistical biophysics group. She was at ICTP on 27 and 28 January to give this year’s Salam Distinguished Lectures, an annual series of talks given by eminent scientists and supported by the Kuwait Foundation for the Advancement of Sciences (KFAS).

Walczak’s lectures focused on the immune system, whose complexity has lured and puzzled scientists for a long time. Data recently made available by experiments have however allowed physicists like Walczak to unveil the rules that underlie the functioning of such an intricate system, thus shedding new light on a fascinating machinery that is crucial to our survival.

In this interview, Walczak tells us what sparked her interest in living systems and what this has taught her about physics. The interview has been edited for length and clarity.

As a physicist, what attracted you to studying problems related to biology?

What really got me excited when I was studying physics is when I discovered statistical physics and the fact that we can describe systems that have a complex behaviour at different scales using coarse-grained phenomenological approaches. I think biological systems seem very complicated, but that does not mean that they do not follow rules. They also necessarily obey physical laws and they therefore provide an extraordinary lens into the physical world. I am interested in understanding how the laws of physics are realized in living systems and in discovering new laws of physics that can emerge from them.
I always liked thinking about living systems, but I found that biology was very descriptive and studying it required learning many names and notions, and that is what initially put me off biology. In a way, statistical physics opened a door for me into a subject that I already found fascinating.

What do physicists bring to biology?

Like biologists, physicists are problem-driven and use different tools to solve their problems of interest, but physicists are more used to quantifying their objects of study and to acquire data in a way that leads to a quantitative description. Physicists tend to search for underlying patterns, even when they are not apparent, and they have been searching for the laws driving the behaviour of complex systems, although they might be difficult to identify. They know that the fact that something seems complicated does not mean that it does not behave according to certain rules, and that in order to find them, we need data. Everything I talked about during my lectures, for example, clearly shows that the immune system follows certain statistical laws. Biologists do not naturally think in those terms, and that is probably why this result was found by physicists.
Physics is ultimately about not only describing but also predicting real world phenomena. Data is essential for that, and once we have started collecting more quantitative information about living systems, they have become a very interesting object of research for us.

What has biology taught you about physics?

One thing that it has taught me is precisely that biological systems follow statistical laws. As I showed in my first lecture, we have found that we can predict the probability of generating an immune receptor and that this is reproducible and universal, but it is statistical.
I am only speculating, but I think that in the future we will find many more statistical laws of this kind.

What led you to studying the immune system?

I first started working on it by chance, but then I have kept on going back to it because it is a problem that encompasses many of the questions that we are interested in as physicists.
The immune system evolves very rapidly, so it provides us with real life data that allow us to look at evolutionary processes at reasonable timescales. It is also an example of a self-organised system, and physicists have long been trying to understand how complex systems can function without the intervention of any external agent. It is statistical, it clearly follows statistical laws. The immune system also encodes many ideas about learning, because it needs to adapt to an everchanging environment; in this sense it is also very similar to a sensory system. Another very interesting property that we as physicists are interested in is the immune system’s ability to differentiate between what belongs to the organism and what doesn’t.
The immune system is essentially a little universe in itself and there are many questions that it can help us answer. Understanding how we are able to protect ourselves from pathogens and why sometimes we fail at that is also an inherently interesting question, whose answer can help us better protect ourselves and create better vaccines.

You also work on other problems, for example flocking. How has your work on the immune system informed your work on these problems?

I work on different problems because they are fascinating and fun and I have great collaborators to work with. I work on them independently and with different collaborators, so although it is true that sometimes ideas about different projects can come together, most of the time I focus on each problem at a time and approach them separately, also because one needs to learn the biology well enough before.

Have you been to ICTP before, and what is your relationship with the Centre?

I have been to ICTP many times. I first came here as a visitor for one week or two, right after moving to France. Later I even co-supervised a PhD student in collaboration with Matteo Marsili. I came back again on several occasions for various conferences and workshops. I really enjoy coming to ICTP and I think it is a great place, with a unique mission, and I know that the events that ICTP has organised in Trieste and across the world have been the starting point of many collaborations.

ICTP has a tradition to hold a private Q&A session with diploma students after lectures from distinguished speakers. How was this experience for you?

It was really great, the students asked many questions, from very focussed, scientific ones, about both biology and physics, to others, more related to what drove me to biophysics, how I pick problems and how I imagine doing what I do forever. I really enjoyed the interaction, so much so that we went a bit over time. What I found really amazing is that essentially all of them asked questions. You should really ask them what they thought about it.