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Flu Season Primer Part II: The Immune System

When we left off last week in “Flu Season Primer Part I: The Virus” your immune system was about to mount a spirited defense to a viral infection. Your immune system protects you from all sorts of threats and invasions, including parasites, bacteria, allergens, insect bites, viruses and more. In this blog I will only address the immune system with regard to defense against viruses.
Just like a mediaeval castle with moats, drawbridges, and ramparts, your body mounts a multi-layered defense to viruses. The first lines of defense are physical barriers. The key physical barrier is your skin. However, since you need to breath, eat, smell, hear, reproduce, eliminate waste and the like, you can’t be completely closed off to the outside world. The various openings in your body are protected by other physical barriers including mucous, saliva, and cilia.
Once your physical barriers are breached your body immediately recognizes the virus as “not you” and your innate immune system swoops in. Your innate immune system is alerted to the infection when your infected cells release compounds called cytokines (cytokines will figure prominently in part III of this series). Cytokines function as the communication network for your immune system. Cytokines send out chemical messages which call in other elements of your innate immune system which can do things like shutting down protein synthesis within your cells so the virus can’t make copies of itself, or engulfing viruses to clear them from the body. The key things to remember about the innate immune system are that it responds immediately to an invasion, the response is not specific to the particular pathogen, and you are born with it.
The third line of defense that you have against viruses, the adaptive immune system, is, by far, the most impressive. Even as a trained scientist there are some natural phenomena which I find unapproachably complex. As an example, consider memory. I can easily conceive of the physical and chemical processes which occur in the brain, but how those processes result in our ability to recall past events is beyond me. For me, the adaptive immune system is in this same class as memory. I’ll do my best to convey that wonder to you as we proceed.
The soldiers of your adaptive immune system are the white blood cells which are produced in your bone marrow. Your body produces several classes of white blood cells. Some fight bacteria, some are involved in allergic reactions, and some, called lymphocytes, fend off viruses. You may be familiar with the names of two types of lymphocytes, B cells and T cells.
All viruses have unique proteins called antigens which are essential to the function of the virus and are also akin to a finger print as an identifier.   During the time that your innate immune system is holding the virus at bay, your body starts to generate custom designed B and T cells to identify the new virus by its antigen and to eliminate the infection. The new custom designed T cells are able to determine which of your cells are infected, latch on to them, and, in an act of self-sacrifice, kill off both themselves and the infected cells. The custom designed B cells learn to generate a protein called an antibody which has exactly the correct chemical and physical structure to enable it to latch on to the antigen of the virus. Once the antibody latches on to the antigen, the virus particle ceases to function and is marked for destruction by another type of white blood cell called a phagocyte.
So consider what your body is doing here. If your body has never seen the antigen of a particular virus before, it studies the structure of the antigen and makes a specific antibody perfectly designed to neutralize the virus. You may have lived more than one hundred years but if your body had never seen that antigen before, it can build a new molecule that it has never made in an entire century of life, the antibody to fight that antigen.  I find that to be fascinating enough but then comes the part I really like.
Once your body fends off a virus, it continues to make B cells which can make this newly learned antibody. These are called memory B cells. Every time you are infected with and fend off a virus your arsenal of memory B cells is increased.   Essentially, every time you are infected with and survive a virus your immune system improves. It’s the best example of “that which does not kill me, makes me stronger” that I can think of.
If a virus that your memory B cells already know about gets past your physical barriers, your adaptive immune system reacts almost immediately and produces a vastly larger number of antibodies than were produced during the first infection. It is this fast and strong response of your adaptive immune system which accounts for your being “immune” to some diseases and is the basis of vaccination.
The mechanics of vaccination have been known for thousands of years. People noticed that survivors of a particular epidemic, for example small pox, were able to care for victims of the same disease during the next epidemic without getting sick again. I could do an entire blog just on vaccines, but the short version is that scientists learned to manipulate virus particles such that they could introduce antigens into the human body while disrupting the virus’ ability to infect the body’s own cells. This allows the adaptive immune system to learn and remember how to make the antibodies without having to survive the disease.
The invention of vaccines must count as one of the greatest achievements in human history. Vaccination has allowed us to eliminate or nearly eliminate from the world, small pox, measles, typhoid, polio, whooping cough, mumps and many other debilitating and sometime fatal diseases. Influenza is a virus. So the question on the table is . . . why are we still suffering from the flu while we have wiped out these other scourges?   For the answer, stay tuned for Part III, next week.
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