Impact of the Immune System on the Brain and Mental Health

For many of the major influences on the brain, signals are sent by T cells that travel in the cerebrospinal fluid (CSF). These T cell signals are relayed to specific regions of the brain by special lining cells of the chamber that holds the CSF. T cell signals sent in the CSF to brain cells can have an impact on many aspects of cognition and behavior. For example, when we are ill with a fever, T cells send signals for the brain to create the “sick feeling” so we will slow down and take care of ourselves. When the infection is over, T cells send a different type of signal, using pulses of secreted molecules, that tells the brain to restart and maintain normal cognition. 

In adults, a small number of new neurons are regularly produced in the memory center of the hippocampus. These new neurons are vital for producing new memories. Research has shown that depression correlates with a decreased production of these neurons, which can lead to the decrease in memory often seen in depression. But it was not clear how this happened. It is now known that signals from T cells can alter the production of new brain cells and therefore increase or decrease memory ability. During depression, T cells signal for fewer new brain cells to be made, which leads to reduced memory. These signals also affect the generalized inflammation throughout the body that often occurs with depression. When the depression is treated by medications, ECT, psychotherapy, etc., the immune signals begin to again stimulate the increased production of new neurons, better memory, and decreased inflammation. Better understanding these signals could lead to entirely new treatments for depression. 

Stress is another situation that involves signaling between the immune system and the brain. While short term stress can be helpful by triggering increased learning and a rise in the production of neurons in the memory centers, long term stress does the opposite and can produce damaging inflammation and decreased memory. Both brain cells and immune cells pick up perceptions of stress. In a mechanism similar to that of depression, T cells send signals during long term stress that produce inflammation and decrease the production of new neurons for memory.  

As immune cells travel throughout the body, they have many opportunities to send signals back and forth with neurons and other supportive brain cells. For example, a T cell—the master immune regulator—can secrete molecular signals directly into tissue that then travel to nerves. When the signal is picked up by the neuron, it can be relayed throughout brain circuits, which can then affect various other organs. This leads to a mechanism whereby an acupuncture needle or electrical stimulus triggers a local T cell—for example, in the wrist tissue, nearby but not in a blood vessel or nerve. When triggered by the acupuncture stimuli, the T cell sends a signal into the tissue that travels to a neuron that is not far away, which then relays another signal through brain circuits causing the acupuncture effect in a distant area of the body.

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