What is lymphopenia?

I had a CBC with diff. in feb. of '08 and my lymphocytes came back 840 the norm. is 1250-3380. Then on jun.6 '08 I had another CBC with diff. and my lymphocytes are down to 640. My doctor said it's not serious but I read online it is and I don't know what to do. What causes it?What should I do?
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