Lymphocytes are a type of white blood cell that play a vital role in your immune system. They help your body fight against viruses, bacteria, fungi, and cancer cells. They are responsible for your immune system having a great memory. After encountering the antigen, some lymphocytes turn into memory cells. 

Lymphocytes represent 20-40 % of your white blood cells. There are two main types of lymphocytes – T cells and B cells – and each has a special function. T cells or T lymphocytes destroy and kill cells that have been infected or cancer cells. Around 10 % of T cells live a very long life by turning into memory cells. They have the ability to recognize previously encountered antigens, foreign substances that stimulate your immune response and therefore respond very quickly. B cells or B lymphocytes are the cells that produce antibodies that then attack antigens, including viruses, bacteria, and cancer cells. Like all white blood cells, lymphocytes are produced in the bone marrow. B lymphocytes are released into the blood straight from the bone marrow, while the T cells need time to mature in the thymus gland. 

A high lymphocyte level is called lymphocytosis. Lymphocytes help fight diseases, so an increase after an infection is normal, and in most cases, lymphocytosis goes away as soon as the underlying condition improves. However, some serious diseases can cause a high lymphocyte count. Hepatitis, HIV, syphilis, and tuberculosis all increase lymphocyte count. People with blood cancers like lymphoma and leukemia – acute and chronic – all have elevated lymphocyte levels. Infections like mononucleosis and whooping cough are also reasons for lymphocytosis.  

A low lymphocyte level is called lymphocytopenia or lymphopenia. It can be caused by bone marrow diseases, like leukemia or anaplastic anemia, where the bone marrow can't make enough lymphocytes. It can also be that your bone marrow is producing a sufficient amount of lymphocytes, but they are being destroyed at a faster rate. This is typically the case in autoimmune diseases like lupus. Lymphocytopenia is also connected to HIV, viral hepatitis, radiation, or chemotherapy treatments.