MODULE SUPPLEMENT: IMMUNE SYSTEM
Overview and Background

The immune system is an organization of cells and molecules with the essential function to differentiate self from non-self in order to protect the body from both external (e.g. infections, toxic products) and internal (e.g. cancer) challenges. It's a dangerous world out there!

It's important to note that the immune system is characterized by:

1. Its complexity
2. The inter-relationships among its various components
3. The inter-relationships it has with other system components; for example:
   1. The immune system is highly inter-related with the neuro-endocrine system.
   2. Nerve endings, especially from the sympathetic nervous system, are found in the thymus, spleen, and lymph nodes, all critical components of the immune system.
   3. As a corollary of this, receptors on immune cells have been found to be responsive to acethylcholine, norepinephrine, and other neuro-hormones
   4. And a number of immune cells are apparently capable of secreting ACTH and endorphin like compounds.

These vast inter-relationships stimulated the evolution of the field of psychoneuroimmunology (Kiecolt-Glaser, et al., 2002).

Unfortunately this vast inter-connectedness also makes interpretation of the data difficult. Very few researchers can study anything but pieces of the whole. Thus conclusions are often too simplistic and too broadly drawn.

One problem with the research is that there are a number of different methodological approaches and these may result in variable findings.

There's also the problem of using animals and of assessing the peripheral vs. central components of the system.  That is, in humans we look at blood components out of practicality while researchers studying animals, especially mice or rats, use the spleen or lymph nodes.