Understand that leukocytes, or white blood cells, are classified based on the presence or absence of granules in their cytoplasm.
Recognize that granulocytes are leukocytes containing visible granules when stained and viewed under a microscope. These include neutrophils, eosinophils, and basophils.
Know that agranulocytes are leukocytes that lack visible granules in their cytoplasm. This group includes lymphocytes and monocytes.
Recall that erythrocytes (red blood cells) and thrombocytes (platelets) are not leukocytes, so they are not part of the leukocyte classification.
Therefore, the two main classes of leukocytes are granulocytes and agranulocytes, based on the presence or absence of cytoplasmic granules.