Immune System

What is it?

A large part of the research carried out at BPRC is concerned with the immune response. Thus, what is an immune response?

Everyday we, and all living animals, face a constant battle to survive, but we are generally unaware of it. Millions of different 'foreign' organisms in the form of viruses, bacteria or parasites constantly threaten our health. In addition cells within our bodies may change into cancer cells. Our immune system has evolved to control these threats.

We become aware of our immune systems when we come into contact with pathogens (dangerous foreign organisms). When all goes well our immune systems find ways to kill these invaders, and although we may feel ill for a while (for instance with the common cold) we recover. Some pathogens are much more dangerous (for instance the AIDS virus, or tuberculosis or malaria), and these pathogens have usually found ways to get around our immune systems.

To understand how we can develop cures for these pathogens we need to understand the workings of the immune system: what does an immune system do?

What does it do?

The immune system is designed to be able to distinguish between self and non-self. It is then designed to make 'made to measure' responses against the non-self (the pathogen) that accurately kill the pathogen, but leave the self untouched. It does this by recognising the molecules of the pathogen and designing other molecules that fit like a lock and key with only the pathogen molecules. These 'designer made' molecules carry with them the tools to kill the pathogen.

The pathogen molecules come in millions of different types. To combat this problem the immune system is extremely complex, and highly efficient so that it can nearly always design new locks to fit these keys. One downside to this efficiency is that each immune system is so finely tuned that it even sees the cells from another person as foreign. That is because our cells carry very small molecular differences from person to person. The immune system is so sensitive that it sees these differences. This means that transplantation of organs to save lives is very complicated. Another problem, due to the complexity of the immune system, is that it sometimes runs out of control and attacks its own body. This is called autoimmunity, and is the cause of a number of very serious diseases.

The components of the immune system all start from 'stem cells' in the bone marrow. These stem cells are very flexible, and can develop into many different cell types. Some may grow into the cells that make special molecules called antibodies, or they may grow into special cells, called T-cells, which patrol your body looking for foreign invaders. Some of these patrolling cells have the task of controlling and helping the immune response, and some have the task of binding to the pathogen and killing it.

What we have learned over the past few years is that the business of identifying non-self is very complex indeed. You can consider your immune system as an organ of your body, just as your heart or your brain, except it is spread over your whole body. It has to be spread, because the pathogens can attack from anywhere (skin, lungs, intestines, etc.). However, the problem with it being spread everywhere is how do you keep the immune system organised?

How is it organised?

The patrolling cells of the immune system constantly report to small bean-shaped lymph nodes spread throughout your body. Here, in particular, the immune cells communicate with one another in special areas. This communication helps to ensure that only responses to non-self molecules are permitted, and to ensure that when a pathogen is present a strong immune response occurs.

Another important function of this communication is to ensure that the immune system 'remembers' each pathogen. This means that if it encounters the pathogen again a much faster and more effective immune response can occur. This works better for some diseases than for others. For example most infections with the measles virus leave a very good immunological memory. This memory reacts to any later encounters with the measles virus and ensures that we do not get infected again. For reasons that we don't fully understand, this does not work with all diseases. For example, in huge areas of the world, children repeatedly get infected with malaria. They do not develop an immunological memory that is strong enough to control later malaria infections.

Detail from a stamp commemorating the start of polio vaccination in 1955 This immunological memory is the reason that vaccines work so well. From the first vaccine ever developed against smallpox, to the vaccines against polio and more recent vaccines such as those against hepatitis B, vaccines have saved many millions of lives all around the world.

Vaccines work by giving the immune system the information it needs to make the 'keys' to fit the molecular 'locks' of the pathogen. However, in contrast to an infection with a pathogen, they give this information in a non-dangerous way.