Leucocytes without clearly visible granules are called agranular leucocytes or agranulocytes. There are two types of agranulocytes: lymphocytes and monocytes.

1. Lymphocytes: Lymphocytes comprises 20-25 percent of the total WBCs. They are usually the second most numerous type of leucocytes. They are small cells with round nuclei and little cytoplasm. They exist in two major forms, namely B and T lymphocytes and are responsible for immune response of the body.

2. Monocytes: They are the largest leucocytes and are amoeboid in shape. They have kidney or horse-shoe shaped nuclei. They spend about 30-40 hours in the blood and then enter the tissues where they become macrophages. Macrophages are phagocytic in nature and engulf bacteria and other cellular debris.

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