White blood cells (WBCs) were identified by William Addison and Gabriel Andral. While Antonie van Leeuwenhoek described red blood cells much earlier in 1674, it wasn't until improved microscopes became available that Addison and Andral were able to identify the far less numerous WBCs.
In more detail:
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Antonie van Leeuwenhoek: He is credited with the first description of red blood cells in 1674.
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William Addison (1802–1881) & Gabriel Andral (1797–1876): They are jointly credited with identifying white blood cells. The advancement of microscopy technology was crucial to this discovery.
In short, though red blood cells were observed earlier, Addison and Andral are recognized for the discovery of white blood cells.
