WBCs, or leukocytes, are part of the immune system and are classified into two main groups: granulocytes and agranulocytes. Granulocytes include neutrophils, eosinophils, and basophils, which contain granules in their cytoplasm. Agranulocytes include lymphocytes and monocytes, which lack visible granules. Each type of WBC has specific roles in immune defense and response to infection.
