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Immune responses through the seasons

Do you remember your last unreachable mosquito bite, how it itched mercilessly?  That sensation means your immune system is hard at work.  When the mosquito releases its specially formulated saliva into your body- including an analgesic to numb you and an anticoagulant to keep the blood flowing- your immune system springs into action.  Immune cells in your blood recognize the mosquito spit as a foreign invader. They launch histamine and other chemicals to attract more immune cells to the site.  Blood vessels swell to accommodate their arrival.  The histamine also sends a tickle through your nerve cells to your brain, leaving you with an inflamed bump and a persistent itch.

By the time you feel it, the mosquito is already gone!  Unfortunately, the inflammatory response does little to deter this kind of invader.  But it is an important first step in the coordinated immune response that might help you kick the flu this winter.

Immune system: Attack!

The “bugs” that cause winter maladies, like the influenza virus or streptococcus bacteria, usually enter the body through mucous membranes in the nose or mouth.  The invaders have a chemical makeup that distinguishes them from your own cells, which triggers a similar immune response as the mosquito spit (with less of the itchy histamine).  As immune cells gather in the affected area, you experience the symptoms of your cold: sniffles, fever, swelling and soreness in the throat.  So what are those cells doing there, besides making you feel sick?  This time, they actually help get rid of the invader.

Some immune cells target bacteria directly.  They swallow them up and destroy them.  Others are more strategic- like assassins scoping the body for a target- they recognize specific components of viruses or bacteria and use antibodies to disable them.    Then they activate, multiplying in number and making more antibodies against the specific attacker.  Some of the activation process takes place in the lymph nodes, a place your doctor is sure to check for pain and swelling when you’re sick.

You are on the road to recovery when the infection is cleared and your immune system settles down, but your immune system “remembers” the battle.  Antibodies for the recent invader remain in your bloodstream for years, so if you caught the exact same bug again, your body would be ready and waiting to shut it down (this is the basis of how vaccines work – interactive link requires Flash).

Long-term relationships with bacteria and viruses

Not all foreign invaders make us sick; in fact, some are delightful (think: frozen yogurt, unfiltered beer, bleu cheese…).  In truth, these innocuous microbes could be dangerous if they made their way out of your gut and into your bloodstream, or if the wrong strain of bacteria snuck into a food.

Viruses, too, can live on in our bodies after our immune systems give up the fight.  Some, like the chicken pox virus, invoke an immediate immune response, but remain in our bodies after the symptoms are gone, only to resurface years later (in this case, shingles).  The human cytomegalovirus, or CMV, lingers in as many as 80% of adults, but rarely causes symptoms in a healthy host.   Viruses must enter our own cells to survive, and some have developed the ability to sneak around the immune system to do so.

Finding the balance

Our immune system has to find a way to fend off the foreign invaders that really would harm us without going overboard and making us feel sick all the time.  Immune cells and antibodies are constantly on the prowl for danger in the bloodstream, scanning broadly, without launching a full-on attack. Every body loses that balance sometimes, whether it’s a mosquito bite that is still inflamed long after the mosquito is gone, getting the sniffles during allergy season, or a full-on immune system battle-an autoimmune disease.


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