Another group of diseases which can be tested for in the lab are the immunodeficiencies. We generally think of immunodeficiencies as such congenital diseases like Severe Combined Immunodeficiency Disease (SCID) in which the patient has no B Cells or T Cells due to a defect in the RAG gene. However, primary or congenital immunodeficiencies are fairly rare; testing will be described in the next section.
Many pathogens have tricky "evasion" mechanisms which prevent a normal immune response, such as secreting certain cytokine-like chemicals, hiding in a cell, or many other ways. The most common and most devastating of these tricks is caused by the HIV virus which causes the acquired immunodeficiency called AIDS. This is due to the fact that the virus chooses to live in our CD4 T Cells. They slowly become depleted, not just because of the virus's activities itself, but because our CD8 Cytotoxic T cells recognize them as infected cells and destroy them.