Microglia are glia of the central nervous system, named from Greek words for small glue. They tend to be smaller than the other types of glia, which can collectively be called macroglia. Their structure depends on the situation. Resting microglia have a small soma and many long and highly branched processes. Active microglia are larger, variably shaped, and usually do not have processes.
Microglia are the macrophages of the central nervous system. Resting microglia monitor the interstitial fluid for abnormalities, which will cause them to retract their processes and become active.
Active microglia migrate toward abnormal areas with a structure and function similar to macrophages elsewhere in the body.
If they encounter foreign cells or abnormal neural cells, they secrete cytotoxic (cell-killing) substances, such as reactive oxygen species.
They also phagocytose (eat) foreign cells, abnormal cells, or debris, and present antigens from the phagocytosed material to lymphocytes.
This helps the processes of adaptive immunity to fine tune the immune response to the abnormality. In addition to these functions, microglia also appear to influence neurons, other glia, and other cells of the immune system, and vice versa, through exchange of a variety of substances.