The thymus is a small but essential organ overlying your breastplate where naive immune cells from the bone marrow, aka the “T Cells” (for thymus), become mature. It is a “West Point” for T-cells which will be selected for their ability to learn the host’s unique password, the MHC-1 (or Major Histocompatibility Complex 1,) that almost every cell in your body possesses. In the diagram below, this “positive selection” takes place in the cortex. MHC-1 will become important later when we discuss fighting cancer and viruses.
Before they graduate, the majority of T-cells are killed off by apoptosis because they react too strongly to the MHC 1 and might escape to cause autoimmune diseases. This “negative selection” takes place in the medulla of the thymus.Until the 1960’s, scientists considered the thymus to be a useless organ, a graveyard for immune cells. This was probably because the organ was withered by the time their older patients went to autopsy.On the other hand, the ancient Greek physician, Galen (AD 129-200) considered the thymus the “seat of the soul” and “that mystical organ.” We now realize that the thymus is literally and figuratively the center of the adaptive immune system. Galen noted that the relative size was greatest as an infant and that after puberty, the organ shrinks. But the “thymic West Point” probably withers because of lack of enrollment from eligible young T-cells, not from self-destruction.
AIDS is a Form of AgingAdvanced HIV infection, aka AIDS, results in a premature burn-out of the host’s immune system. That is why patients monitor their “T-cell count” as a gauge of how much immune function remains. CD28 (+) cells are “naive T-cells” and are thus able to adapt to new immune threats. With AIDS, the immune system loses its capacity to respond to new threats as the proportion of CD 28(-) (pronounced ‘cee-dee-negative’) T-cells increases. CD28 (-) T-cells, stretching the military metaphor, are 4F, or not suitable for military service.
Previously an extremely rare cancer occurring in elderly Mediterranean men,Kaposi’s sarcoma was one of the first phenomena of AIDS before scientists had identified HIV in the early 1980’s. So what is the relationship between cancer, viruses and immune deficiency?You body is constantly fighting new cancers and latent viruses like an army waging war on many fronts. When all the T cells (West Point officers) and the Natural Killers (enlisted men) are used up, the war is lost.
Definition of AIDS-related cancers: Certain cancer types that are more likely to occur in people who are infected with the human immunodeficiency virus (HIV). The most common types are Kaposi sarcoma and non-Hodgkin lymphoma. Other AIDS-related cancers include Hodgkin disease and cancers of the lung, mouth, cervix, and digestive system.-(excerpted from the NIH’s National Cancer Institute website)