Short answers, pulled from the story.
White blood cells, scientifically called leukocytes, are cells of the immune system that protect the body against infectious disease and foreign entities. They are produced in the bone marrow from hematopoietic stem cells and circulate through the blood and lymphatic system. They make up approximately 1% of total blood volume in a healthy adult.
The normal white blood cell count in a healthy adult is between 4,000 and 11,000 cells per microliter of blood in the United States, or equivalently between 4 billion and 11 billion cells per liter. A count above that upper limit is called leukocytosis; a count below the lower limit is called leukopenia.
The five main types of white blood cells are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils are the most abundant, making up 60-70% of circulating leukocytes. Basophils are the rarest, accounting for less than 0.5% of the total count.
Granulocytes have visible granules in their cytoplasm and a lobed nucleus, while agranulocytes lack prominent cytoplasm granules and have a round nucleus. Neutrophils, eosinophils, and basophils are granulocytes; monocytes and lymphocytes are agranulocytes. White blood cells can also be classified by lineage into myeloid and lymphoid cells.
The term leukocyte comes from the Greek roots "leuk," meaning white, and "cyt," meaning cell. The name white blood cell itself derives from the physical appearance of a blood sample after centrifugation, where white blood cells collect in a thin, typically white layer called the buffy coat, visible between the sedimented red blood cells and the blood plasma.
Leukopenia is most often due to a drop in neutrophils, called neutropenia, which can be triggered by chemotherapy, antibiotics, anticonvulsants, radiation, alcohol, benzene exposure, or underlying conditions such as AIDS, rheumatoid arthritis, and bone marrow failure. A decrease in lymphocytes, called lymphocytopenia, is defined as a total lymphocyte count below 1.0 billion per liter and can result from infections like HIV, autoimmune diseases, zinc deficiency, or major surgery.