A major component of the Lymphatic system . In the thymus lymphoid cells undergo a process of maturation and education prior to release into circulation. This process allows T-cells to develop self tolerance (distinguishing self from nonself). While developing in the thymus gland, any T-cell that reacts to the thymus' major histocompatibility complex (MHC) is eliminated. T-cells that tolerate the MHC learn to cooperate with cells expressing MHC molecules and are allowed mature and leave the thymus.
The result is that surviving T-lymphocytes tolerate the body's cells and cooperates with them when needed. However, some T-lymphocytes lose this ability to differentiate self from nonself, which results in the autoimmune diseases such as systemic lupus erthematosus or multiple sclerosis.