Posts Tagged ‘immune system’

The Adaptive Immune System

January 31, 2009

This is a continuation of the previous post about the immune system, where now we focus on the lymphatic system.

Humans have a very advanced part of their immune system known as the adaptive immune system, which is only found in jawed vertebrates.  In this system certain leukocytes, called Antigen-presenting cells (APCs), can bring in another line of defense by notifying T-cells that there is an infection.

How they accomplish this is quite remarkable:  by tearing by apart the internalized pathogen and presenting the pathogen’s antigens on its surface.  Antigens are structures which identify the pathogen and produce a specific immune response in T-cells.  Our APC must now move to our T-cells, which are located in our lymph nodes, and present the antigens to activate them. The APC again uses chemotaxis to traverse through lymph vessels to reach the nodes.  It presents the antigens attached to a special structure called MHC to “naive” T-cells.

t-cell2

This T cell (blue), one of the immune system’s principal means of defense, identifies the molecular signature of a dendritic cell (gray) at a contact point called the immunological synapse. http://www.lbl.gov/Science-Articles/Archive/PBD-immune-system.html

T-Cells have special T-Cell Receptors (TCRs), which have constant parts which always bind to MHC, and variable parts which will only bind to specific antigens.  Upon activation some T-cells become “T helper cells” (CD4+) and others become “Cytotoxic T-cells” (CD8+). The CD4+ cause the growth of more CD4+, which will release chemicals which stimulate more white blood cells to the site of the infection.  Cytotoxic cell kill infected cells, as their name implies.  They both return to the site of the infection through chemotaxis.

After the infection is eliminated, the specific antigens which the T-cells bound to will be “remembered” by memory T-cells.  These cells bind only with the previous antigen and so the immune system is more prepared for a recurrence of the same infection.

Clearly I glossed over much, so read more for the real details.  It’s an amazing system all accomplished with molecular machines!

The Common Cold

January 27, 2009

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.