Page 640 - Veterinary Immunology, 10th Edition

VetBooks.ir Neural Regulation of Immunity

The nervous and immune systems communicate by multiple
pathways. They communicate through parasympathetic and

sympathetic nerves and by soluble neurotransmitters.
Neuroendocrine hormones such as corticotrophin-releasing factor
and α-melanocyte-stimulating hormone, as well as some
neurotransmitters, act on cells of the immune system to regulate
cytokine balances. Conversely, cytokines and chemokines modulate

nervous system activities such as appetite, body temperature, and
sleep behaviors. There are transport systems that carry cytokines
across the blood-brain barrier. In addition, inflammation in tissues

can lead to activation of sensory nerves that then relay messages to
the brain. Many of the cytokines produced in the brain are
produced by brain and meningeal macrophages—microglia. For
example, IL-1 is produced in the brain in response to endotoxin.
Memory T cells may secrete acetylcholine in response to vagal

nerve stimulation. Electrical stimulation of the vagus inhibits
cytokine release and reduces inflammation-mediated injury in
endotoxemia and sepsis. This is called the inflammatory reflex and

requires acetylcholine production.

Stress

Stress influences immunity (Fig. 20.17). Small bouts of stress are
believed to enhance immune responses, but prolonged stress is

detrimental. One obvious example is shipping fever. This is a
complex pneumonia of cattle caused by several viral respiratory
pathogens with secondary infection by Mannheimia hemolytica. It
develops in cattle that have been transported in confined spaces for

long distances (and hence, many hours) with minimal feed and
water and usually after rapid weaning and castration. The stress
involved in the shipping process is sufficient to make these cattle
highly susceptible to pneumonia. Stress depresses T cell responses,

NK cell activity, IL-2 production, and expression of IL-2R on
lymphocytes. Reduction in stress can have a reverse effect. In
studies investigating innate responses among calm and

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