Lymphocytes are white blood cells that travel in the blood stream and defend the body from abnormal cells, disease-causing bacteria and viruses. Large Lymphocytes, called natural killer (NK) cells, recognize abnormal cells and destroy them by releasing toxic chemicals. Small Lymphocytes, the T cells and B cells, recognize foreign proteins called antigens that are found on bacteria and viruses and then release toxic chemicals to destroy them.

Blood tests that show Lymphocyte numbers as higher than normal usually mean the presence of a virus infection. For example, when someone has AIDS (acute immune deficiency syndrome) or HIV (human immunodeficiency virus) infection, the T cells are destroyed by the invading virus until the body cannot protect itself from other infections. This person is vulnerable to getting very sick even from the common cold because T lymphocytes are not available to provide the needed protection.

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