Lymphocytes are the cells of the body that are involved in the immune mechanism. These are the commonly known white blood cells.
The most common reason for a decreased lymphocyte count is an underlying viral infection. As more and more lymphocytes will be involved in fighting off the infection, the count of the lymphocytes would be low when tested.
As the viral infection is totally combated by the body, the count will be recovered in a matter of a few weeks.
The point of concern in this case is the infection caused by HIV. With this virus in the body, 'opportunistic' pathogens (that do not usually cause an infection in a normal individual) will cause an infection in the body of the patient.
Another reason for a low lymphocyte count is aplastic anaemia. In this condition, the bone marrow where the lymphocytes are made does not function properly and thus the count of the lymphocytes is decreased.
There are certain hereditary conditions where the bone marrow does not produce a normal amount of the lymphocytes, thus resulting in a low count of lymphocytes.
Some treatments and drugs cause a drop in the number of lymphocytes too. People who are undergoing radiation therapy, chemotherapy or taking steroids will have a low lymphocytes count making them more susceptible for infections.
Some neurological diseases can also cause a decreased lymphocytes count. Diseases such as multiple sclerosis (MS) and Guillain-Barre Syndrome are examples of such neurological diseases.
Auto-immune disease such as rheumatoid arthritis will also cause a dramatic drop in their count.
Certain types of cancers, especially the ones concerning the blood and lymph, can also cause a low lymphocytes count.
So a low absolute lymphocyte count can be due to an infection, drugs, therapy or some diseases.
A differential blood count performed in lab can detect a low absolute lymphocytes count.
If a person's lymphocytes count is slightly low, then the test can be repeated after some time and it might not be an indication of a serious problem. But if the count is too low, then a serious problem could be the cause of the low lymphocytes count.