Biochemistry is always more complex than you think. Via Medical News Today, we learn that our stem cells are very involved in the immune response: "marrow stem cells -- undifferentiated cells that eventually give rise to the blood cells that fight infection -- possess receptors that recognize bacteria and viruses. When activated, these receptors kick the stem cells and immature blood cells into action, enlisting them to help fight whatever pathogen is attacking the body." This opens the door to possible methods of controlling, enhancing or repairing the aging immune system. "It may be possible to boost immunity when necessary and also shut down inappropriate responses. That could provide a powerful tool to fight cancer, lupus and many other diseases."