Neutrophils
Neutrophils, seen here, play a central role in the immune system. They are one of the first cell types to respond to activated endothelial cells and to migrate into infected or inflamed tissue. They are also the most abundant cells in the blood, making up around 2/3 of all leukocytes. Activated endothelium can therefore quickly mobilise a whole army of fighters to the infected site! But this is not the only thing the activated endothelial cells do. The initial process of neutrophil adhesion also activates, or "primes", the neutrophil, making it optimally armoured for the coming fight. Did you see how the neutrophil was glowing about 1 min 30 sec into video 3? This was because adhesion molecules on the neutrophils had been activated and were sending signals. These signals made granules within the neutrophils fuse with the cell membrane and we saw the release of molecules that made an opening in between endothelial cells where the neutrophils could squeeze through. But what we didn’t see was how this fusion of granules also resulted in exposing various new receptors on the neutrophils surface. These new receptors are like new tools for neutrophils. New adhesion molecules further strengthen the adherence to the endothelium so that the blood flow doesn’t flush them off. Chemokines receptors now make it possible for the neutrophils to "smell" the invader through the endothelium and move towards it like a sniffer dog. Other receptors help the neutrophils recognise and ultimately phagocytose pathogens, like complement- and Fc-receptors. Take a look at this video to get an understanding for how excellent, persistent and vicious the neutrophil "sniffer dog" is!
Neutrophil priming also initiates the production of superoxide products and their release in the respiratory burst. Once in the infected area it is the respiratory burst along with phagocytosis that are the main weapons that neutrophils strike with. After this quick outburst and heroic act the neutrophils die in the tissues within one to two days. But who will tidy up this battlefield of dead neutrophils? Remember the resident house keeper? And what if the invader was not defeated? Who will help then?