Helminths (parasitic worms) are too big to be phagocytosed. Eosinophil granulocytes, when released from their home in bone marrow, circulate in peripheral blood, and then traffic to tissue, particularly in the lungs and gut, which are the mucosal surfaces where helminths may reside.
Eosinophils encountering a helminth put on a fireworks show: ROS respiratory bursts to blast holes in worm cell membranes, as well as releasing cytotoxic proteins.
When not on worm duty, eosinophils fight viral infections, help deal with inflammation, and mediate allergic reactions and asthma pathogenesis. Eosinophils also get involved in many other biological processes: female breast development, the estrous cycle, allograft (transplant) rejection, neoplasia (abnormal cell growth) response, and antigen presentation, as part of the acquired immunity response.
Eosinophils are an example of an evolutionarily older mechanism being upgraded to take on additional duties in a highly coordinated and integrated system.