In this blog, we will see how the immune response needs energy, how is this energy demand met by the body and how are the other functions minimized to meet the energy demand of the immune response. This demand and overall attempt of the body to minimize other biological functions to meet this demand again shows the need of the interconnection of the nervous system and the immune system.
Energy is a limiting resource which must serve all physiological processes. Body requires energy to mount an immune response that could otherwise be allocated to other biological functions like growth, reproduction. E.g. every 1ͦ C increase in body temperature during fever increases oxygen consumption by 7–13%. Antigens and pathogens stimulates more robust immune responses more energetically costly. This includes the production of acute phase proteins and all of the cellular proliferation, production of cytokines, collagens, proteases, and so forth, require a large supply of amino acid building blocks.
The cytokines, such as IL-1, IL-6, and TNF, are key coordinating elements, both peripherally and through their ability to communicate with the CNS. At the cellular level, IL-1 promotes the breakdown of muscle protein into amino acids, a process that is responsible for the muscle soreness experienced during infection. The action of pro-inflammatory cytokines, with IL-1 the most potent pyrogenic agent, causes fever. IL-1 also increases the availability of glucose for metabolism by peripheral tissues and the release of fatty acids from fat stores for similar use. IL-1 and other cytokines produce a set of behavioural changes like decreases in activity, exploration, social interaction, and food and water intake. All of these are also considered to be part of the acute phase response.
Balancing the demand:
During infection, calories must be made available for the increased energetic demands of the immune system. Acute infective illnesses are accompanied by a cluster of non-specific symptoms like
- An increased need to sleep, (reduce unnecessary energy expenditure)
- Hyperalgesia (cause reduce activity and direct behaviours such as licking to the site of inflammation)
- Anorexia (systems evolved must forage to find food and water and that digestion is energy intensive. IL-1 and TNF slow digestion.)
- loss of interest in usual activities,
- decreased social interaction and body care
- depression and impaired concentration
All of the behaviours are reduce unnecessary energy expenditure, making it available to fight infection or injury. For e.g. increased sleep, particularly slow wave sleep, should reduce the brain’s glucose demand, the brain being the body’s major user of glucose.
Involvement of CNS
The intense energy demands of inflammation and the acute phase response shifts entire energy balance in an organism. This can only be accomplished by involving the CNS, so that the changes from metabolism to behavior can be orchestrated. Hence it is important that immune products communicate with the CNS. IL-1, TNF, and IL-6 are key elements of the communication. The behaviour changes in response to IL-1 produced during immune response may have evolved to conserve energy.
During acute stress:
During acute stress low-energy-consuming immune components are enhanced and high-energy-consuming ones are suppressed. Cells and antibodies that are already prepared to act are mobilized into the saliva and the blood; such mobilization comes at a relatively low energetic cost. At the same time, cell production in the blood, which comes at a relatively high energetic cost, is inhibited. The consequence appears to be a maximization of immunity while minimizing energy expenditure.
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Maier (1994) Psychoneuroimmunology, The Interface Between Behavior, Brain, and Immunity. American Psychologist. Vol. 49. No. 12, 1004-1017.
Demas (2004) The energetics of immunity: a neuroendocrine link between energy balance and immune function. Hormones and Behavior 45:173–180.