Various forms of glial cell exist in the brain, supporting and protecting neurons. Over the years, researchers have discovered that glial cells are deeply involved in many of the important functions of neurons, such as the establishment and maintenance of synaptic connections. Some forms of glial cell, such as microglia, are a part of the innate immune system. They differ in many aspects from similar types of immune cell elsewhere in the body, macrophages, but have much the same set of responsibilities: clean up debris; consume pathogens; destroy errant cells; assist in regeneration from injury. In the aging brain, immune dysfunction sets in similarly to the rest of the body. Immune cells become overly activated, inflammatory signaling grows, but at the same time the immune system becomes less capable of carrying out its core tasks.
Of late, the research community has devoted increasing attention to the balance of states in microglia and macrophage populations. These cells have a number of overlapping states, or polarizations, a way of characterizing their behavior. The M1 state is less helpful in regeneration, and more inflammatory and aggressive in pursuit of pathogens. The M2 state, on the other hand, suppresses inflammation
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