What is A reduction in the total leucocyte count is called: and explain in detail?
Leukopenia is a condition where a person has fewer white blood cells in their bloodstream than they should. Leukopenia is diagnosed with a blood test called a complete blood count or CBC. 
A healthy white blood cell count is between 3,500 and 11,000 white blood cells per microliter. A person with leukopenia may have fewer than 3,500 white blood cells per microliter. 
White blood cells are made in the bone marrow and are critical for the immune system. Having too few of them means the body is less able to fight off infections and diseases.