How does stress affect the immune system?

The researchers of this study analyzed more than 300 articles describing a relationship between psychological stress and how it affected the immune system in human participants (Segerstrom SC, Miller GE.,2004).
They found that chronic stressors were associated with suppression of both cellular and humoral immunity.
A cellular immune response activates phagocytes, white blood cells that engulf pathogens, and also, antigen sensitized cytotoxic T-cells.
Humoral immunity is carried out by substances like antibodies and complement proteins found in extracellular fluid.

Vaccinations are used to trigger an antibody response to provide immunity, but not everybody responds the same way to vaccinations.
This research was conducted to examine the influence of psychological stress on antibody responses following influenza vaccination (Pedersen AF, et.al., 2009).
The researchers analyzed 13 studies and found that there were significantly lower antibody responses related to higher stress levels.

This shows how important it is to be able to stay in a good state of mind even if we are exposed to stressful situations.
Regular meditation is a good way to condition yourself to do that.

References:

Pedersen AF, Zachariae R, Bovbjerg DH. Psychological stress and antibody response to
influenza vaccination: a meta-analysis. Brain Behav Immun. 2009 May;23(4):427-33.

Segerstrom SC, Miller GE. Psychological stress and the human immune system: a meta-analytic
study of 30 years of inquiry. Psychol Bull. 2004 Jul;130(4):601-30.