= Blood cell
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>.<Blood Tests by the National Heart Lung and Blood Institute>[Complete blood count, red cells, white cells, platelets and haemoglobin]{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.<Means for counting particles suspended in a fluid by Wallace H. Coulter>[US2656508A, description of Figs.1–4 and 6–7; dilution and masking]{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 $5000\times1000/1=5{,}000{,}000$ 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.
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