What Causes a High Lymphocyte Count?

A high lymphocyte count, or lymphocytosis, can be caused by mononucleosis, multiple myeloma, human immunodeficiency virus, cytomegalovirus infection, acute lymphocytic leukemia, chronic lymphocytic leukemia, vasculitis and other viral infections.

What Is a Lymphocyte? Lymphocytes are white blood cells found in the immune system, and there are two main types. These are B cells and T cells, and they combine their efforts to fight off infection. Antibodies are produced by the B cells to attack toxins, viruses and bacteria. Any cells in the body that have become cancerous or overcome by viruses are destroyed by T cells, according to Healthline. In adults, a lymphocyte count significantly higher than 3,000 lymphocytes in a microliter of blood is generally considered to be lymphocytosis, notes the Mayo Clinic.

There are different types of B cells and T cells. Effector cells combat active infections when activated by antigens. Memory cells have the ability to recall past infections. If an antigen re-infects the body, memory cells are quickly activated.

B cells transform into plasma cells when an antigen is recognized, producing antibodies. T cells comprise cytotoxic, helper and regulatory T cells. Cancer cells, cells infected with an antigen or foreign cells, such as those from a transplant, are killed by cytotoxic T cells. The immune response of the other T cells and the B cells is directed by the helper T cells. The immune system is suppressed by the regulatory T cells so that perceived or actual antigens are not attacked by other white blood cells.

Where Do Lymphocytes Originate? B cells and T cells come from bone marrow, originating from stem cells. Roughly 25 percent of the new lymphocytes stay in the bone marrow, transforming to B cells. The majority, however, move to the thymus. This is where they transform into T cells. Lymphocytes are continually produced by bone marrow, with most moving through the organs and tissues that comprise the lymphatic system, while some enter the bloodstream.

Obtaining and Understanding a Lymphocyte Count A blood test can determine the level of lymphocytes. This is called a B and T cell screen. It is possible to have a high lymphocyte count and display no symptoms, notes the Mayo Clinic. If test results indicate a high lymphocyte count, a person’s doctor may perform additional tests to determine if the lymphocyte count is harmless and temporary or if it signals a more serious underlying issue, such as blood cancer or a chronic infection. These tests may include a peripheral blood smear, a lymph node biopsy, a bone marrow biopsy or an IgE level measurement. The latter tests the level of a specific antibody concentrated within the blood.

Because lymphocytes are an essential part of the immune system and help to rid the body of disease, it is not uncommon for a high lymphocyte count to occur after an illness, states the Mayo Clinic. Once the illness has passed, the count will return to normal. It is rare that a high lymphocyte count is found randomly because it is normally discovered after a doctor has ordered tests to assess a pre-existing condition. Additional testing may not be required if the high lymphocyte count coincides with test results the doctor has already obtained.