We know that the body’s immune system is important for keeping everything in check and protecting us. But a lot of us don’t realize that the brain also has an immune system.
Microglia, which are the brain’s immune system cells, are involved in everything—from brain development to protecting against diseases such as
meningitis and
Alzheimer’s. But for all the good these cells do for us, under the wrong conditions they can also cause us harm.
Microglia belong to a group of non-neuronal cells called glia, which originally were thought to play a supportive role for the brain’s neurons. Now research shows that microglia actually do much more than only support the neurons: they nourish, protect, and sometimes even destroy them.
These cells are unique because they come from the same place as other immune system cells, but have a different origin from other brain cells, which develop from neural stem cells. Microglia come from the
yolk sac—an extra embryonic membrane—and travel to the brain
early during its development.
Microglial functions are especially crucial during brain development, when they
help young neurons grow and ensure the right connections are made between neurons. Through a procedure called
pruning, microglia eat connections between neurons, maintaining strong ones while eliminating weaker or unnecessary ones. This continues somewhat during adulthood. For example, microglia remove unimportant memories by
eating or altering synapses involved in their maintenance.
Faulty pruning during brain development has been linked with disorders such as
schizophrenia and
autism. But uncontrollable pruning by microglia in adulthood has been implicated in developing diseases such as
Alzheimer’s.
Microglia have specific receptors on their surface that recognize distress signals from other cells. These signals
attract microglia to the site of the problem. When the brain’s balance is disturbed (usually as a result of inflammation), living neurons can become stressed and
produce these signals. This may cause them to be eaten alive by microglia. As neurons are killed, the connections they have with other neurons are also eliminated, which can cause severe issues in brain connectivity and functions.
Inflammation causes microglia to change roles, and turn into their aggressive form to defend the brain. Usually, when stress signals stop and anti-inflammatory signals are received, microglia go back to first repairing, then
protecting the brain.
But there are cases, such as with
chronic stress,
aging, and
neurodegenerative disorders, where microglia can become more aggressive and less easy to regulate, making them more dangerous for the brain. In these cases, microglia can increase in numbers, unnecessarily kill nearby cells, and may contribute to making the brain even more inflamed by secreting inflammatory molecules. They also
don’t go back to their protective role easily.
Look After Your Brain
But there are many things we can do to keep our microglia happy—and our brains healthy.
Sleep: Microglia never sleep, but they clean and repair the brain and improve memory while you do.
Lack of sleep, however, has also been shown to make microglia take on their inflammatory form.
Although we know some things about microglia, we don’t know everything. We know some things about how they form, that they’re involved in many diseases and that they might essentially control the brain. But we also know we can’t control them. Future research might focus on how we can stop microglia from causing diseases, and how to stop these cells from turning against the brain.