>
<p>
Both of these white blood cells are involved in fighting off bacterial 
  infections. 
</p>
<p>
They also found that another type of cell that tackles viruses and tumours 
  increased with age, with women having a higher rate of increase than men. 
</p>
<p>
They also found that men showed a decline in two types of cytokines that help 
  to keep the immune system under control and prevent inflammation from 
  damaging surrounding tissue. 
</p>
<p>
Professor Hirokawa added: &ldquo;It is well known that 