Leukopenia is a decrease in the level of leukocytes in peripheral blood below 4000 in 1 µl. The cause of this condition can be infectious, inflammatory autoimmune diseases, genetic defects of the immune system, etc. Clinically, leukopenia is manifested by an increased susceptibility of the body to various infections, but it can be completely asymptomatic. The level of leukocytes is measured when taking venous or capillary blood in the morning on an empty stomach. To correct this laboratory deviation, the underlying disease is treated.