Stress and ageing: effects on neutrophil function

The innate or non-specific immune system consists of soluble components, namely the complement system and cellular elements. The latter includes neutrophils, which make up the majority of the innate immune cells in circulation, and deal with rapidly dividing bacteria. This chapter will focus on the influence of psychological stress and ageing on neutrophil numbers and function. Neutrophils are a major component of innate immunity and are the dominant leukocyte in the circulation, making up 60 % of the white cell count. They are also the shortest lived blood cell, dying by apoptosis approximately 24 h after leaving the bone marrow (Savill et al., J Clin Invest 83:865–875, 1989; Scheel-Toellner et al , Biochem Soc Trans 32:461–464, 2004). These cells play a crucial role in killing invading pathogens, particularly rapidly dividing bacteria, and are key cellular components of the early phase of inflammatory responses (Nathan et al., Nat Rev Immunol 6:173–182, 2006). Neutrophils act quickly and without specificity, although their bacterial recognition systems are many and complex. Neutrophils are recruited to sites of infection via chemical homing (chemotactic) signals, such as the chemokine CXCL8 (also known as IL8). Once in contact with the pathogen they uptake the microbe by engulfing (phagocytosis) mediated via opsonic receptors that detect complement proteins C3b and C3Bi or antibody coating the microbe.