Granule (cell biology)

In cell biology, a granule can be any structure barely visible by light microscopy. The term is most often used to describe a secretory vesicle.

Leukocytes
A group of leukocytes called granulocytes contain granules and play an important role in the immune system. The granules of certain cells, such as Natural Killer Cells, contain components which can lead to the lysis of neighboring cells. The granules of leukocytes are classified as azurophilic granules or specific granules. Leukocyte granules are released in response to immunological stimuli during a process known as degranulation.

Platelets
The granules of platelets are classified as dense granules and alpha granules.