T cell deficiency is a medical condition in which the body's immune system is weakened due to a lack of T-lymphocytes. There are several synonyms for this condition, including: T lymphocyte deficiency, T cell immunodeficiency, T cell lymphopenia, and T cell dysfunction. All of these terms describe a situation in which the body's T cells are either absent or unable to function properly. T cell deficiencies can be caused by a variety of factors, including infections, genetic disorders, and certain medications. Treatments for T cell deficiencies may include immune-modulating drugs or stem cell transplantation, depending on the underlying cause of the condition.