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A blood cell is a cellular component of blood, including red blood cells and white blood cells. Red blood cells transport oxygen using hemoglobin; white blood cells participate in the immune system. The usual blood-count categories also include platelets, which in humans are cell fragments rather than complete nucleated biological cells.^[ref] Conversely, a mature human red blood cell is a whole specialized biological cell without a cell nucleus; possessing a cell nucleus is not a universal test for membership.

The Coulter counter connects cell counting to electric current. Wallace H. Coulter's patent passes a diluted suspension through a narrow conducting path between electrodes. A particle changes the electric current as it passes, yielding a countable pulse. The known sampled volume and dilution convert counts to an estimate for the original sample; simultaneous passage of particles can mask events.^[ref]

For a synthetic arithmetic example, count 5,000 cells in $1\mu \mathrm{L}$ of a sample diluted by a factor of 1,000. The original sample then contains an estimated $5000\times 1000/1=5,000,000$ cells per microlitre. This illustrates counting and dilution, not a clinical reference range. A pulse alone does not establish which cell type produced it; the preparation and classification method must specify that.

A Coulter counter measures suspended particles by detecting electrical pulses as they cross a constricted conducting path. A particle with different electrical properties from the suspending fluid changes the electric current. Counting pulses in a measured volume gives an estimate of particle abundance, with dilution used to recover the value for the original sample.^[ref]

Wallace H. Coulter's patent, filed in 1949 and granted in 1953, describes two insulating vessels, electrodes and a narrow passage, with detection circuitry registering particle events. It also identifies masking when particles occupy the sensitive region together.^[ref]

For a synthetic example, 2,500 accepted pulses in $50\mu \mathrm{L}$ of a 100-fold diluted suspension implyThe estimate assumes representative sampling and correctly counted events. A pulse is not automatically a blood cell of a particular type: debris, coincident passage and classification thresholds affect what is counted.

Wallace H. Coulter was an engineer and inventor who developed electrical counting of suspended microscopic particles, the principle behind the Coulter counter. His patent application was filed on 27 August 1949 and granted as US2656508 on 20 October 1953; these are different events in the history of the apparatus.^[ref]

The apparatus passes an electrically conducting suspension through a small aperture between electrodes. A particle crossing that restricted path changes its electrical behavior and produces a detectable pulse. Counting pulses in a measured sample volume estimates the number of particles; known dilution connects that estimate to the original sample. The setup replaces the need to identify and count every particle visually with a repeatable electrical measurement.^[ref]

For example, 2500 accepted pulses in $50\mu \mathrm{L}$ after a hundredfold dilution imply $2500\times 100/50=5000$ counted particles per microlitre in the original suspension. This synthetic example assumes that particles are well mixed and individually resolved. Two particles passing together can be confused with one event, so a counter requires control of sample concentration as well as electronic sensitivity.