12 curiosities about the immune system

Few structures in the body are as complex and, at the same time, as astonishing as the immune system. To mark the launch of the CaixaResearch Institute, which is dedicated to the study of immunology, we explore some of the stories, oddities and discoveries that reveal just how extraordinary our defences truly are.

The first description of immunity

Long before the existence of microscopes, vaccines or even a clear understanding of what infections were, some civilisations had already observed that certain diseases were only experienced once, or that if they returned, they did so in a much milder form. One of the earliest written references to this phenomenon was made by the Athenian historian Thucydides. In his account of the Plague of Athens in 430 BC, he noted that those who had recovered from the disease were able to care for the sick because they did not contract it again. That observation anticipated one of the fundamental principles of immunology: the body’s ability to remember an infectious agent. It would take almost two thousand years for that ancient intuition to become a real medical tool, but the idea was already there, recorded in Antiquity.

Smallpox: the first time we learned how to protect ourselves

At the end of the 18th century, smallpox was one of the deadliest diseases. In this context, the English physician Edward Jenner took note of a common belief among milkmaids: those who had contracted cowpox, a mild infection transmitted by cows, seemed to be protected against smallpox. Intrigued by this observation, Jenner decided to put it to the test in 1798 by carrying out an experiment that would change the course of medical history. He inoculated a young boy with material taken from a cowpox lesion and later confirmed that he was protected against smallpox. This experiment marked an unprecedented breakthrough: for the first time, humanity was not merely observing immunity, it was learning how to induce it deliberately.

Humanity has succeeded in eradicating a pathogen

The eradication of smallpox remains, to date, a unique achievement. Following decades of mass vaccination campaigns, epidemiological surveillance and international cooperation, the last naturally occurring case of smallpox was recorded in Somalia in 1977. Three years later, in 1980, the World Health Organisation officially declared the disease eradicated. Never before had humanity succeeded in eliminating from circulation a human pathogen of such impact. However, this success also has a more troubling dimension: with the end of universal vaccination, the global population has gradually lost its immunity to smallpox. For this reason, although the virus no longer circulates naturally, it is still considered a potential threat in scenarios involving accidents or bioterrorism.

The first vaccine came from the cow

The word vaccine has a much more literal origin than many people realise. It derives from the Latin vacca, meaning “cow”, and refers directly to the pioneering work of Edward Jenner, who used cowpox to immunise and protect people against smallpox. Years later, Louis Pasteur built upon this legacy and expanded it. By studying attenuated microorganisms – that is, those that have been deliberately weakened – he observed that they could induce protection without causing severe disease, and he chose to call this technique vaccination in honour of Jenner. This work laid the foundations of modern immunology research and led to the creation of leading scientific institutions such as the Pasteur Institute in Paris, which remains one of the world’s most prestigious centres for the study of infectious diseases and the development of vaccines.

Vaccinated individuals protect those who cannot be vaccinated

So-called herd immunity, also known as collective immunity, is one of the most important concepts in public health. It occurs when a sufficiently large proportion of the population becomes immune to an infection, either through vaccination or prior exposure, making it more difficult for the pathogen to find susceptible individuals to infect. As a result, transmission slows, and those who cannot be immunised are indirectly protected. It is not a perfect shield, nor does it work in the same way for all diseases, as it depends on factors such as how contagious the pathogen is. Nevertheless, it represents a highly effective collective barrier. Its importance lies in a fundamental idea: protection is never purely individual, it is also communal.

Vaccine safety, backed by science

Despite the vast body of evidence accumulated, the false link between vaccines and autism remains one of the most persistent health myths. Its origin dates back to a study published in 1998 that suggested a connection between the MMR (measles, mumps and rubella) vaccine and autism. That study not only had a very small sample size and serious methodological flaws, but was also ultimately retracted due to fraud, data manipulation and conflicts of interest. Since then, numerous studies conducted in different countries, involving hundreds of thousands of children and using far more robust designs, have conclusively ruled out any link between vaccination and autism spectrum disorders. The scientific consensus is unequivocal. What is concerning is that this myth, despite being thoroughly discredited, continues to have real consequences: it undermines confidence in vaccines, lowers vaccination rates and contributes to the resurgence of preventable diseases that were once thought to be under control.

Influenza never stops changing

Unlike more stable pathogens, influenza viruses are constantly changing. They do so through frequent mutations in their surface proteins, particularly those recognised by the immune system. This means that the defences developed against a previous strain may not be as effective against variants circulating months later. For this reason, the influenza vaccine is reviewed and updated each season based on the strains that, according to international surveillance, are most likely to predominate that year. Annual vaccination, therefore, is not due to a lack of effectiveness of the vaccine, but rather to the ever-changing nature of the virus itself.

HIV, one of the most complex viruses

HIV has an extraordinary capacity to mutate, enabling it to change rapidly and evade immune recognition. It also specifically targets key cells of the immune system itself, weakening the body’s defences from within. In addition, it can integrate itself into the genome of infected cells and remain hidden in reservoirs for years, making it extremely difficult to eliminate. All of this explains why, despite decades of intensive research, we still do not have a fully effective preventive vaccine, although progress in this field continues steadily.

Our mothers gave us immunity

Not all immunity is acquired after contracting an illness or receiving a vaccination. There is also passive immunity, which involves receiving antibodies already produced by another organism. One of the most important examples occurs the relationship between mothers and their children. During pregnancy, the mother transfers antibodies to the foetus through the placenta, and after birth she can continue to provide protection through breastfeeding. These antibodies provide the newborn with a crucial temporary defence at a stage when their own protective mechanisms are still immature.

The microbiota, a community that protects us

An essential part of our health depends on living alongside microbes. The human body hosts trillions of microorganisms (bacteria, viruses, fungi and archaea) that form complex communities in the gut, skin, mouth and other mucosal surfaces. This collective, known as the microbiota, plays a role in digestion, produces useful compounds, contributes to metabolism and plays a decisive role in the development and training of the immune system. Exposure to these microorganisms helps our defences learn what should be considered harmful and what should not, and when this balance is disrupted, inflammatory, metabolic or allergic problems may increase. In other words, a part of our defences is built through the constant dialogue with the microbes that live within us.

Fever, pain and inflammation are defence strategies of the body

When someone has a fever, muscle pain or inflammation, it is common to assume that all these symptoms are caused directly by the invading pathogen. However, in many cases, much of the discomfort actually comes from the body’s own immune response. When an infection is detected, the body triggers a complex cascade of chemical signals that coordinate the defence. Fever, for example, raises body temperature to hinder the replication of many pathogens and enhance certain immune functions. Similarly, inflammation helps recruit defensive cells and direct them to the site of infection. In this way, what we experience as illness is often the visible trace of a body actively fighting to protect itself.

Our immunological arsenal can fight against what does not yet exist

One of the most remarkable characteristics of the immune system is its ability to anticipate the unknown. This is possible because certain immune cells, such as B and T lymphocytes, randomly generate millions of different receptors, each with a slightly different shape, capable of recognising specific molecular structures. Thanks to this vast diversity, the body has a repertoire ready to detect not only known pathogens, but also entirely new ones. Rather than waiting to encounter each threat and then designing a response from scratch, the body maintains an immense library of possibilities already prepared for action.