'WBCs' originates from modern English as an initialism formed from the phrase 'white blood cells'.

'WBCs' was created in 20th-century medical and laboratory contexts as a convenient abbreviation of the phrase 'white blood cells' (which itself comes from 'white' + 'blood' + 'cell').

Initially, the full phrase 'white blood cells' referred to any pale-staining cells in blood; over time the term and its abbreviation have come to specifically denote leukocytes — the immune cells in blood.