The leukocytes, also known as white blood cells, protect the body against infectious and harmful diseases. These diseases are usually caused by bacteria, infections, and parasites. The leukocytes are classified based on their lineage, structure, and types of white blood cells. Based on their structure, the leukocytes are classified into granulocytes and agranulocytes. Based on lineage, they are classified into lymphoid cells and myeloid cells. Based on the type of white blood cells, they are classified into eosinophils, neutrophils, lymphocytes, monocytes, and basophils.