Infections, leukemia and arthritis can cause too many white blood cells, according to Mayo Clinic. Certain medications and smoking can also cause this condition.
The main role of white blood cells is to protect the body against certain infections. If an infection is present, the white blood cells multiply in order to destroy the infection. A blood test can determine whether a person has a high white blood cell level, as stated by WebMD. This can help doctors indicate whether or not an infection is present or if there is another condition that may need treated. White blood cell counts are usually measured during a complete blood count.