cientists, whose research is published in the journal <i><a href="http://www.immunityageing.com/">Immunity 
  and Ageing</a></i>, examined the blood of 356 men and women aged between 20 
  and 90 years old. 
</p>
<p>
They looked at levels of white blood cells and cytokines, which help to carry 
  messages in the immune system. 
</p>
<p>
In both sexes the number of white blood cells decreased with age, but two key 
  elements &ndash; known as the T-cell and B-cell lymphocytes, declined faster in 
  men. 
</p