Is too low a level of white blood cells caused by an antibiotic?

You did not write exactly what antibiotic "from tetracyclines" your daughter takes. In general, in the case of tetracyclines as a group, haematological disorders as an adverse reaction cannot be excluded. Due to the fact that leukopenia (too low level of leukocytes) can have many causes, it is also impossible to clearly confirm that the result is caused by the drug. With the result of the morphology, you should go to the doctor - he will check which cell line is responsible for the general low level of leukocytes, collect an anamnesis and order additional tests, if any. I enclose materials that may be helpful: https://www.gdziepolek.pl/artykuly/fakty-i-mity-o-antybiotykach https://www.gdziepolek.pl/artykuly/jak-i-dlaczego-zglaszac-dzialania-niepozadane https://www.gdziepolek.pl/artykuly/jak-przygotowac-sie-do-badania-krwi