a source of something produces that something & specialized cells are a source of chemicals that destroy disease-causing microbes ->  specialized cells produce chemicals that destroy disease-causing microbes  decreasing something negative has a positive impact on a thing & destroying something decreases the amount of that something ->  destroying something negative has a positive impact on a thing  destroying something negative has a positive impact on a thing & disease-causing microbes have a negative impact on the body ->  destroying disease-causing microbes has a positive impact on the body  destroying disease-causing microbes has a positive impact on the body & protecting a living thing has a positive impact on that living thing 's survival / health ->  destroying disease-causing microbes is a kind of method for protecting the body  specialized cells produce chemicals that destroy disease-causing microbes & destroying disease-causing microbes is a kind of method for protecting the body -> specialized cells protect the body be producing chemicals that destroy disease-causing microbes