Stress and immunity

The brain helps the spleen make antibodies

Kirill Stasevich, Science and Life (nkj.ru ) based on Nature News: Brain–spleen connection aids antibody production

One of the most important functions of the spleen is to help immune cells synthesize antibodies. As is known, antibodies are produced by plasmocyte cells, which are obtained from B-lymphocytes. In order for B-lymphocytes to turn into plasmocytes, they must receive a signal from T-lymphocytes, which inform them that something alien has appeared in the body. But T-lymphocytes must first receive information about what exactly appeared in the body – that is, they must "see" a foreign molecule that indicates, for example, a viral infection.

And here we need another immune cells, which are called antigen-presenting. They pick up a foreign molecule or a fragment of it (this is the antigen that needs to be presented) and show it to T-lymphocytes in a special way, and they already notify B-lymphocytes; at the same time, immune cells weigh all the pros and cons – whether it is really necessary to declare an alarm, or there is no serious danger. And so the spleen provides a place, or better to say, an environment where signals are exchanged between immune cells.

In the spleen, as in any other organ, there are nerves – these are the nerves of the autonomous nervous system, which works independently of the brain. However, as the experiments of researchers from Tsinghua University have shown, the work of the spleen depends not only on the autonomic nervous system, but also on some parts of the brain.

Article by Zhang et al. Brain control of humoral immune responses amenable to behavioral modulation is published in the journal Nature.

If the nerves transmitting signals from the brain were removed from the spleen of mice, and then an antigen was injected into the animals, which was supposed to provoke the synthesis of antibodies, then their level did not increase.

In order for B cells to begin to turn into plasmocytes synthesizing antibodies, they must be affected by acetylcholine, a molecule that is usually called a neurotransmitter; it plays an important role in signal transmission between neurons of the autonomic nervous system. But acetylcholine is also secreted by T lymphocytes, which have sensed a foreign antigen and activated - with the help of acetylcholine, they activate B cells. At the same time, T-lymphocytes secrete acetylcholine for a reason, but under the action of the neurotransmitter norepinephrine. That is, in order for T cells to stimulate B cells, they must sense the antigen and receive a norepinephrine signal.

Norepinephrine T cells receive from splenic neurons, which, in turn, receive impulses from other neurons transmitting signals from the brain itself. An article in Nature says that signals to the spleen are sent by the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala (or amygdala). The hypothalamus is one of the main neuroendocrine regulators in the body, controlling a variety of processes, while the amygdala is called the center of emotions, which helps to make decisions in difficult situations.

Groups of neurons that send signals from the hypothalamus and amygdala to the spleen are known to control the stress response when an individual feels fear and threat. Some of these neurons secrete the hormone corticotropin, from which the stress response begins – and it is the corticotropin neurons in the nuclei of the amygdala and hypothalamus that send signals to the spleen. If corticotropin-synthesizing neurons did not work, then no new plasmocytes appeared in the spleen.

On the other hand, under stress, neurons of the same amygdala and hypothalamus stimulate the adrenal glands to secrete the hormone glucocorticoid, which suppresses the immune system. It turns out that at the same time under stress, the immune system is stimulated and suppressed? The authors of the work write that everything here depends on the strength of stress: only moderate stress stimulated the synthesis of antibodies – apparently through signals from corticotropin neurons to the spleen. This can be understood in such a way that a stressful situation generally increases the risk of infection, and antibodies are needed to fight infection, but with severe stress, first of all you need to deal with the most stressful situation, and immunity can wait.

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