4. Regulation of immune responses
Acquired immunity as described in section 2 of this appendix is vital to protect the body against infectious disease, but these immune responses are potentially dangerous if they are not properly regulated as happens in those who develop allergies and autoimmune diseases
Preventing or limiting inappropriate immune responses, is brought about by T lymphocyte cells called regulatory T lymphocytes (Treg cells) that have an ‘immune management’ role.
Under certain conditions, T reg cells secrete anti-inflammatory substances which suppress the helper Th2 and Th17 cell responses leading to downregulation of allergic and autoimmune reactions., thereby avoiding development of allergic and autoimmune diseases
The Old Friends mechanism
In normal humans, T cell secretion of the anti-inflammatory substances that suppress allergic and autoimmune responses is driven by exposure to Old Friends organisms such as helminths, commensal microbiota and environmental saprophytes. (They also help to switch off responses to infections once the antigens have been eliminated).
In the absence of stimuli from these Old Friends, Treg cells are no longer adequately induced causing increased susceptibility to “overreaction” leading to allergic and autoimmune diseases.
Over time the body has evolved to distinguish antigens of disease-causing microbes that need to be eliminated, from antigens of the OF organisms that need to be tolerated, but which are still recognised by the immune system and interact with it to drive the Treg responses which prevent allergies and autoimmune disease.
It may be that constant exposure to a biodiverse library of harmful and non harmful microorganisms, and self antigens is necessary for maintaining and constantly evolving our immune regulatory system.