Lymphocytopenia.

http://en.wikipedia.org/wiki/White_blood_cell

Defined as total lymphocyte count below 1.0x10⁹/L, the cells most commonly affected are CD4+ T cells. Like neutropenia, lymphocytopenia may be acquired or intrinsic and there are many causes.This is not a complete list.

• Inherited immune deficiency - severe combined immunodeficiency, common variable immune deficiency, ataxia-telangiectasia, Wiskott-Aldrich syndrome, immunodeficiency with short-limbed dwarfism, immunodeficiency with thymoma, purine nucleoside phosphorylase deficiency, genetic polymorphism
• Blood cell dysfunction - aplastic anemia
• Infectious diseases - viral (AIDS, SARS, West Nile encephalitis, hepatitis, herpes, measles, others), bacterial (TB, typhoid, pneumonia, rickettsiosis, ehrlichiosis,sepsis), parasitic (acute phase of malaria)
• Medications - chemotherapy (antilymphocyte globulin therapy, alemtuzumab, glucocorticoids)
• Radiation
• Major surgery
• Miscellaneous - ECMO, kidney or bone marrow transplant, hemodialysis, kidney failure, severe burn, celiac disease, severe acute pancreatitis, sarcoidosis,protein-losing enteropathy, strenuous exercise, carcinoma
• Immune dysfunction - arthritis, systemic lupus erythematosus, Sjogren syndrome, myasthenia gravis, systemic vasculitis, Behcet-like syndrome, dermatomyositis,Wegener granulomatosis
• Nutritional/Dietary - alcohol abuse, zinc deficiency

Like neutropenia, symptoms and treatment of lymphocytopenia are directed at the underlying cause of the change in cell counts.