Brain coordinates physiological, immunological, and behavioral responses
to stress. A typical stress response is “freeze, fight or flight”. It involves the release of
catecholamines by the sympathoadrenal system and activation of the hypothalamopituitary-
adrenal (HPA) axis to produce various hormones that prepare the organism
for a particular response. Stress generally leads to a strong suppression of the adaptive
immune system and a partial suppression of the innate immune system. The brain
controls the immune organs through hormones produced by the HPA axis and through
direct control by neurotransmitters. In turn, immune mechanisms affect behavior and
brain development. Brain microglia support developing neurons, phagocyte dead and
inactive cells, and participate in the formation of connections between neurons through
synaptic pruning. Peripheral immune cells regulate neurons through neurotransmitters
and inflammatory mediators such as IL6, TNF, PGE2, histamine, and others. These
mediators induce sickness behavior during illness. They also alter the developing brain
during maternal stress, maternal immune activation, and early life stress predisposing
the brain to a mental illness. Immune mechanisms contribute to psychiatric and
neurodegenerative disorders.
Keywords: Alpha 7 nicotinic acetylcholine receptor, Blood-brain barrier,
Corticotropin-releasing hormone (CRH), Depression, Hypothalamic-pituitar-
-adrenal (HPA) axis, Hypothalamus, Inflammatory reflex, Microglia, Nociceptors,
Neurodevelopment, Neurodegenerative diseases, Neural reflex, Psychological
stress, Proopiomelanocortin, Synapse pruning, Sickness behavior, Vagus nerve.
