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Cell counting estimates the number of biological cells in a preparation. A count per calibrated volume gives number density; multiplying by the whole preparation's volume estimates its total count. A trypan blue exclusion test adds an operational distinction between dye-excluding and dye-stained biological cells.

HeLa is a human continuous cell line originating in cervical cancer biological cells collected from Henrietta Lacks in 1951. Its name combines the beginning of her first and last names.^[ref]

ATCC's CCL-2 HeLa preparation grows attached to a surface. Its product sheet specifies a cell culture medium supplemented with fetal bovine serum, at $37{}^{\circ }\mathrm{C}$ with $5\%$ carbon dioxide in air.^[ref] These are specified conditions for that preparation, rather than defining conditions for every HeLa derivative.

Continuing access to related biological cells makes HeLa useful for comparisons of cell state and responses to interventions. But a continuous cell line is a laboratory model: its history and environment matter, and observations in it do not alone establish responses in an intact organism. Cell counting measures abundance; a trypan blue exclusion test measures a particular membrane-associated readout; neither proves cell line identity.