Basophils are present in blood. They contain granules of histamine and other inflammatory mediators. In response to infection or allergens, basophils degranulate , meaning that they release the granule contents by exocytosis. The release of histamine and other factors leads to inflammation and other symptoms associated with allergic reactions and infections. Mast cells are almost identical to basophils.
Mast cells are present in tissues.
Eosinophils are granulocytes that play a role in defending against parasitic worms. They secrete their granule contents onto worms, which helps kill them. They also play a role in allergic asthma. Monocytes are found only in blood. When a monocyte enters tissues, it differentiates into a macrophage.