White blood cells, or leukocytes, are the cells your body uses to fight off infections. There are different types of white blood cell, each responsible for fighting different microbes. For example, neutrophils help fight off bacterial infections, while lymphocytes increase during viral illnesses. According to the Mayo Clinic, the definition of what is a low white blood cell count varies with the age and sex of the child.
The bone marrow generates the body’s supply of white blood cells, so any condition that affects the bone marrow can reduce the number of cells in the blood stream. For example, overwhelming bacterial infections can destroy neutrophils faster than they are produced, according to the Merck Manuals. Viruses can penetrate the bone marrow and affect the production of white blood cells. Parvovirus, a common infection in children, can cause a condition called aplastic anemia, shutting down production of cells, including the white blood cells, inside the bone marrow. Infection with the HIV virus also causes a decrease in the number of lymphocytes, making the child susceptible to serious infections.
Conditions Present at Birth
Some children are born with genetic defects that prevent them from making white blood cells. Most congenital immune deficiencies are X-linked, meaning that only boys are affected. Children born with severe combined immunodeficiency, the so-called “Bubble Boy Syndrome”, lack a specific type of white blood cells, crippling their immune system. According to the NIH, without a bone marrow transplant, these children usually die of opportunistic infections by their first birthday. Children with another condition, cyclic neutropenia, have a periodic decrease in their neutrophils count, followed by periods of normal cell numbers. During the low count periods, these children are susceptible to bacterial infections.
Low white blood cell counts can also occur as a result of a child taking medicines or undergoing certain therapies. Children with cancer undergoing chemotherapy can have low blood cell count, as the chemotherapy wipes the bone marrow clean of the cells that produce the white blood cells. Corticosteroids used by children with chronic conditions like lupus can suppress the bone marrow production of white blood cells. And cancers like leukemia can crowd the bone marrow, decreasing its ability to make white blood cells.