How Does the Immune System Exert Selection Pressure?
Our immune system is extraordinary. Like a 3-D printer on steroids, we create antibodies in response to trillions of novel antigens (which, by definition, are substances that elicit an immune response). We are constantly challenged with antigens. I might be shopping at the local Food City in the midst of flu season (and haven’t yet gone for vaccination). As I stand in the checkout line with my milk and orange juice, a shopper coughs. I inhale, and flu virus forced from her lungs enters into mine. My innate immune system kicks into gear. Like Homeland Security this is the system that patrols my body—ready to act against threatening foreign objects no matter flu, pathogenic bacteria, or some other unfamiliar entity. Cells belonging to this system will engulf the offender and release an arsenal of chemical defenses. If it’s a viral infection, “natural killer cells” will arrive, creating a sort of firebreak by causing cell death. I may run a fever and drop into bed with muscle pain. This collateral damage is the cost of nonspecific defense, but it buys my body time to initiate a more specialized response by way of the adaptive immune system.
Most of us are familiar with antibodies produced by the adaptive immune system, but it also produces “killer cells.” Both are exquisitely specific for antigen. Each cell of this adaptive system is recruited from a huge storehouse of immune cells just waiting to be selected—much as I imagine Amazon or Netflix warehouses carry an unimaginable array of books or movies. When we come into the world we are equipped with a universe of immune cells bearing receptors formed through a process of genetic recombination. Some of these are called B cells. These are the cells that make antibodies. When B cells interact with antigen they become activated and undergo a process of mutation and selection. Those producing antibodies that best fit the pathogen (in this case the flu) survive—a sort of Hunger Games for immune cells. Most will produce ineffective antibodies. Some produce antibodies against our own antigens—not a good thing—and those die early in the process. The cells that produce the most-effective antibodies are cloned. The process continues, ratcheting up the specificity and refining the antibody. It can be a week or two before the adaptive immune system is fully effective—which means that sometimes viruses evolve the means to escape immunity; or that we may be feeling the effects of infection for a while before the system kicks in. It’s a lot of work but well worth the effort. Once exposed, the adaptive system provides us with immune memory. Long after the offending pathogen is gone, whether bacterial or viral, a set of immune cells will circulate, acting as sentries and responding far more rapidly, days rather than weeks, should the same, or similar, invader come knocking again.