I have moderate leukopenia &absolute neutropenia &ralative lymphocytosis .what is the cause?

Multiple causes: You have a low level of one kind of white blood cell, the neutrophil. Most common cause is viral infection, esp. If transient. Many different medication can do this as a side effect, including non-prescription meds. Rarely, nutritional deficiencies (iron, B12 or folic acid). It can be part of autoimmune disease, it can be congenital, be due to serious blood diseases, or be benign.