The Common Cold

A few weeks ago had a small cold.  I wondered what was really going on inside my body.  It turns out this is a complicated and fantastic story about the immune system.  First let’s over the high-level events of the common cold:

  1. A Rhinovirus infects the upper respiratory system
  2. Leukocytes detect the virus
  3. Leukocytes initiate inflammation
  4. Adaptive immune system is activated
  5. The virus is eliminated

Now let’s check these steps out in more detail:

Infection

A virus (usually a rhinovirus) enters your nose and lands on your adenoids (part of your tonsils) the virus binds to the epithelial cells with proteins called cell adhesion molecules.   You can think of it like cellular Velcro, but instead of little plastic hooks they’re made of protein structures which interlock.  This binding changes the structure of the virus’s protein shell (the capsid), which in turn causes myristic acid to be released onto the infected cell.  This acid creates a pore through which the virus can inject its RNA.  Once RNA is within the host cell, it begins replication through RNA transcription.

Detection

The immune system has cells which contain special “pattern recognition receptors” (PRRa) made of protein, which bind with various pathogens including viruses.   Examples of these cells are leukocytes (white blood cells) including macrophages and dendritic cells which both bind with viruses.  If a leukocyte binds with a rhinovirus (or other pathogen) with its PRRs, then it will internalize it in a process called phagocytosis.

Here is an incredible video of another leukocyte called neutrophils chasing down a bacterium and phagoticizing it.  Neutrophils is about 14 microns across, and I believe this video was shot over several hours:

Inflammation

These activated leukocytes respond to the binding of pathogens by release chemicals called cytokines.  These chemicals are one way which cells communicate and organize their efforts.  In the immune system, releasing cytokines, as well as other chemicals, draw more leukocytes to the site of the infection through an amazing process of cellular motion called chemotaxis.   Chemotaxis is basically proteins moving in response to their chemical environment, which causes a cell to propel itself  in a specific direction to reach its chemical goal.  This process escalates the inflammation in a feedback loop of chemotaxis, phagocytosis, and cytokine release.  These released chemicals also cause the familiar symptoms of inflammation: redness, heat, swelling, pain, and loss of function.

In my next post I’ll talking about the adaptive immune system which is pretty incredible.

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