White blood cells can be categorized into two main types – granulocytes and agranulocytes. They are differentiated based on the presence or absence of protein granules in the cytoplasm. Each type is further divided into subtypes, each of which plays a slightly different role in the immune system. 

Granulocytes

Granulocytes make up the bulk of the white blood cells. They are also known as granular leukocytes or polymorphonuclear leukocytes. Granulocytes contain enzyme granules or sacs that form in the cytoplasm of the cell. On receiving a signal of an infection or inflammation, granulocytes rush to the site and release their granules to fight the infection. 

Granulocytes are categorized into 3 distinct subtypes - neutrophils, eosinophils and basophils. Each type has a distinct function that is different from the others. This is because each type of granulocyte contains a unique combination of enzymes and chemicals in its granules. 

• Neutrophils are involved in eliminating bacteria and fungi by phagocytosis. 
• Eosinophils help destroy parasitic and cancer cells. 
• Basophils are primarily associated with combating allergic reactions by releasing histamine. 

Agranulocytes

Agranulocytes are distinguished by the absence of distinct granules in their cytoplasm. They are further categorized into two subtypes – lymphocytes and monocytes. 

• Lymphocytes: One of the most abundant and also the most important types of white blood cells, lymphocytes are responsible for producing antibodies that defend the body against harmful pathogens. 
• Monocytes: The largest of the white blood cells, monocytes play a vital role in cleaning up the debris at the injury site after the infection has been eliminated.