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The process that initially allows immune cells to destroy pathogens is called phagocytosis. This first step is essential in immune response, since it not only allows for a reduction in total pathogen numbers but also allows a proper identification of the invader in order to organize a proper immune response.

The immune response will include the activation of other systems, like the nervous system and the immune system, in order to elicit an adequate response. However, communication with the brain is complicated since it is usually isolated by the blood-brain barrier. Therefore, cytokines have to take alternate routes like the vagus nerve.

Once in, the brain cells must respond.  We usually think of neurons as essential, however it is another group of cells called glial cells that are the first ones to respond. Astrocytes are probably the most complex glial cell; they are really sensitive to cytokines and can in turn control the release and metabolism of most neurotransmitters. Immune-brain communication is an essential part of our physiology - for example, by disrupting glutamate we may feel confused, tired or have memory problems.

As a result of the neurotransmitter imbalance we can perceive a myriad of physical and mental manifestations. Amongst these, anxiety and depression are very common.  Current research suggests that low grade inflammation could be associated with clinical depression, and new anti-inflammatory therapies have shown promise in treating depression and opening the door to treat other mental illnesses.