The immune system is a defense system in our bodies that fights against foreign substances that cause diseases and illness. Immune system consists of different forms of cells that kill bacteria and other substances. Macrophages are large immune cells that engulf foreign substances such as bacteria.
The autonomic nervous system keeps our bodies functioning at a homeostatic level. The immune system and the nervous system interact to fight off any bacteria, viruses, and other foreign substrates that can harm our bodies. The brain and immune system are connected through the vagus nerve and sympathetic nerve fibres to the spleen, liver, bone marrow, thymus, lymph nodes, skin, and the digestive system. These are the main sites of the immune system. The immune and nervous system are also connected by chemical messengers such as small molecules (nitric oxide), neuroendocrine peptides, and proteins such as cytokines.
The immune system signals the brain by signaling the hypothalamus. To get a signal to the hypothalamus, the immune system must release its soluble signalling chemicals to be able to pass through the blood-brain barrier which is a tightly secured barrier to the brain. This function causes a rise in body temperature (fever) to fight diseases. This function is also used to regulate body temperature.
The nervous system communicates with the immune system by sending signals to immune cells that have the specific receptors for the neurotransmitters that are released by sympathetic or parasympathetic nerves. The neurotransmitters which are chemical signals, bind to their receptors that are on the surface of the immune cells. This causes an immune response based on either sympathetic or parasympathetic activity.
References:
Steinman, L. (2004). Elaborate interactions between the immune and nervous systems. Nature Immunology,5(6), 575-581. doi:10.1038/ni1078
M, K. N., Białymstoku. (1999). [How does the immune system communicate with the brain?]. PubMed. Retrieved July 23, 2018, from https://www.ncbi.nlm.nih.gov/pubmed/10540726.
Kenney, M. J., & Ganta, C. K. (2014). Autonomic Nervous System and Immune System Interactions. Comprehensive Physiology,1177-1200. doi:10.1002/cphy.c130051