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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Lymphocytes ( Figure below ), a type of white blood cell, are the key cells of an immune response. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the body's lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs. Answer: lymphocytes
Output:
What type of cells make up about one quarter of all white blood cells?