Final answer:
A hemocytometer is used to count white blood cells, specifically for manual counts in a lab setting. Flow cytometry is utilized for automated and specific cell counts with fluorescent labeling. Centrifuges and spectrophotometers are not used for cell counts.
Step-by-step explanation:
The instrument used to count white blood cells (WBCs) is A) Hemocytometer. A hemocytometer is a specialized counting chamber designed for performing cell counts. It consists of a thick glass microscope slide with a rectangular indentation that creates a chamber. This chamber is etched with a grid of precise dimensions that allows for an accurate estimation of cell concentration within a given volume of liquid. When performing a differential white blood cell count, a technician makes a blood smear and stains it with a specific dye, such as Wright-Giemsa stain, and then manually counts the different types of white blood cells under a microscope using a hemocytometer.
Alternatively, for automated and more specific types of cell counts, such as determining the levels of CD4 T cells in an HIV patient, flow cytometry is used. In this technique, a sample containing white blood cells is treated with fluorescently labeled antibodies, which bind specifically to certain cell types. The cells are then passed through a flow cytometer where they are individually illuminated by a laser, causing the attached fluorogens to emit light that is then detected and used to count and categorize the cells.
While a centrifuge is used for separating components of the blood, such as during a hematocrit test, and a spectrophotometer is used to measure the absorbance or transmission of light by a liquid sample, neither is designed for counting cells.