As is so often the case in biology, what looks like a simple process – see bacteria, catch bacteria, kill bacteria – is actually quite complex. Neutrophils are one variety of a kind of white blood cells called granular leucocytes, which form a major part of the immune system. The cell membrane of each neutrophil is studded with chemical receptors. These receptors are specialized to detect the proteins that immune cells release when they encounter an infection or inflammation. When neutrophils receive such calls for help, they follow their chemical gradient back to the source with great speed. Upon arrival, foreign bacteria are destroyed through phagocytosis – the neutrophils engulf and then digest the invaders. The bacteria do get revenge of a sort. Eventually the neutrophils die, and are themselves phagocytosed and subsequently turned into pus, which isn’t a particularly glorious end for such staunch defenders.
Neutrophil (yellow) engulfing anthrax bacteria (orange). Scale bar is 5um. From PLoS Pathogens Vol. 1(3) November 2005.