Eosinophils
The eosinophil, seen here, is another kind of inflammatory cell that migrate from the blood into the infected or inflamed tissue.
They have become a hallmark for parasite infections as these tissues are swamped with eosinophils.
So what do the eosinophils do to attack the parasite?
Take a look at the video snippet below and see if you can identify the eosinophils’ main weapon and how it was activated or triggered to attack. Notice how this parasite was covered in antibodies. It was when the Fc-receptors on the eosinophil bound the antibodies that the eosinophil degranulated and showered the parasite with its granule content. But eosinophils can also be activated via TLRs binding PAMPs or via complement receptors. Either way; the main weapon for the eosinophil is degranulation. When eosinophils degranulate they release many different mediators like cytokines, chemokines and prostaglandins but above all they release huge amounts of the toxic basic proteins MBP, EPO, ECP and EDN.
Can you remember from the degranulation page what these mediators do? The toxic nature of MBP with its ability to punch holes in membranes will clearly be helpful in fighting off the parasite, but how would the ribonucleases EPO and EDN be useful as it is highly unlikely that the parasites’ RNA will be exposed for attack by the eosinophil? Maybe they are weapons for other invaders? How about a virus? Yes, despite being the hallmark for parasitic infection, eosinophils are also very important in combating viral infections and now you know how! But how do you think the surrounding healthy tissue cells feel when they are exposed to these toxic mediators? Well interestingly there is a theory that the damaging nature of the eosinophils’ mediators might even contribute to the development of asthma.
A link between respiratory syncytial virus (hRSV) infection in young infants and the development of asthma in later life has been found. It is possible that the toxic nature of the eosinophils’ products released to combat the virus damages the airways to such an extent that this starts the extensive airway remodelling seen in asthma. Many other allergic diseases are also characterised by increased number of eosinophils. One reason for this may be that sites of allergic disease often contain lots of the Th2 cytokine IL-5, that probably is the most important cytokine for eosinophil differentiation, activation and survival. Th2 cytokines like IL-5 would also be present at sites of parasite infection. Maybe the eosinophil migrated into the allergic tissue expecting to find a parasite to fight?