What is a granulocyte?

A granulocyte is defined as a type of leukocyte, or white blood cell, that contains granules in its cytoplasm. These granules are made up of various substances that play a critical role in the immune response. Granulocytes can be sub-divided into three main types: neutrophils, eosinophils, and basophils, each having specific functions in fighting infections and responding to inflammation or allergic reactions. Understanding granulocytes and their roles in the immune system is fundamental for grasping how the body defends itself against pathogens.

The other options describe different biological concepts that do not pertain to granulocytes. The smooth prominence of the frontal bone refers to a part of the skull, while a brain tumor is a growth within the cranial cavity, and a hormone produced by the pancreas relates to the endocrine system's regulation of metabolism. These do not have any association with the classification or characteristics of white blood cells.