Monocytes are the largest of all leucocytes. They play major roles in the immune system of our body such as phagocytosis of invading cells, production of cytokines, etc.
Basophils are leucocytes which play a major role in inflammatory or allergic reactions. They are responsible for the production of inflammatory mediators like histamine, which bring about the inflammatory or allergic reaction.
Eosinophils are stained by acidic dyes like eosin.
Neutrophils are also known as polymorphs because they have a multi-lobed nucleus.