The immune system is a complicated and mysterious thing. We understand the basics of how it works, but every week we discover more and more about just how fine-tuned and complex it really is. From treating asthma and allergies to immunotherapy designed to target cancer cells, studying the immune system can really change the face of public health over the next decade or so. However, just understanding how the immune system functions on a daily basis is a colossal task. Immune cells patrol the blood vessels and tissues in the body, swiftly clearing out any intruders such as parasites of bacteria. They are also trained to recognize cells that have gone rogue and that are in the process of forming a tumor or of spreading throughout the body. This constant surveillance is what makes cancer so relatively rare – nipping it in the bud before it really has time to develop.
For cancer to successfully grow, cancer cells therefore need to develop the ability to hide from immune surveillance. They do so through a bewildering array of different strategies. One of the most common is by modifying their antigen presentation. Cells around the body display specific molecules that signal to the immune system who they are and what they’re doing. These molecules are known as antigens and they are in many ways much like the flags on a ship, identifying its nationality and purpose. When something goes wrong in a cell, this is usually reflected in the types of antigens that are exposed on its surface. To continue our naval metaphor, when pirates seize a ship they usually display a different, skull-and-bones type of flag. This is a signal that the ship, or rather the cell, has gone rogue and a clear indicator to the coast guard, or the immune system, that the flag-bearer must be sought and destroyed. However, much like the most cunning pirates, cancer cells often find ways to go around this safety mechanism. They do so by displaying “normal” antigens, therefore hiding in plain sight right under the unsuspecting nose of the immune system. Much like a pirate ship flying a standard flag, cancer cells can go about their deadly business undisturbed and wreak havoc wherever they end up.
Another exciting way in which cancer cells can hide in plain sight is by manipulating the immune system. Cancer cells often find ways to produce invisible chemical signals that lead the immune system to shut off or simply to turn a blind eye to the growing colony of malignant cells that is developing in the body. An accurate if colorful analogy is the classic Star Wars:A New Hope scene where Obi One Kenobi convinces the imperial Stormtroopers that “these are not the droids they are looking for”. Instead of the Force, cancer cells use complex chemical signals that lower the guard of immune cells, allowing them to go forth and, sadly, multiply. Cancer cells also manipulate the immune system in other ways, ensuring its cooperation and actually making it work to help cancer spread. (Why the Jedis never used the Force to turn all the Stormtroopers against the Emperor is an entirely separate question).