Hi, My 2 Years Old Son Got His Blood Test

Hi, My 2 years old son got his blood test results back - it's the third one in the last 2 months with low count of Lymphocytes - the doc called me and said that it is now 4.3 (the previous ones were lower) and that the lowest border for normal results should be 5. She wants to run it to a pediatric specialist and got me worried. my son hasn't been sick the last 2 months and I wanted to ask how serious does this sound and what could be the reasons?