Live attenuated virus vaccines (measles, mumps, rubella, varicella, bacille Calmette-Guérin) could be lethal in individuals with T cell defects and should not be administered. The replication of vaccine viruses can be enhanced in persons with immune deficiencies, such as leukemia, lymphoma, generalized malignancy, or patients under therapy with alkylating agents, antimetabolites, radiation, large doses of corticosteroids. Case reports have linked measles vaccine and measles infection to subsequent death in severely immunocompromised children.

The immune system is classically divided into two responses, the innate and the adaptive responses. The innate immune system is comprised of neutrophils, macrophages, natural killer cells, and complement proteins, which respond rapidly to infections in a manner that is relatively nonspecific to any particular infection. The adaptive immune system is composed primarily of T and B cells, and typically responds to infections more slowly than the innate immune system. The adaptive immune system is much more specific to particular infections than the innate immune system due to a mechanism known as memory.