Ok,
If you get a stomach bug/food poisoning etc, then the body speeds up peristalsis, giving you diarrhoea, and can cramp the stomach muscles to cause vomiting. My question is, how does the body become aware it ha the problem? What detection system is in use here?
For diarrhoea it’s a little more complicated than just speeding up the movement of the gut. There are several mechanisms for diarrhoea.
Changes in the absorption of water, irritation and inflammation of the gut can all cause diarrhoea.
Bacteria most commonly associated with food poisoning produce exotoxins which cause the symptoms. I believe that viruses cause diarrhoea through irritation and inflammation of the gut, they invade cells to replicate and this would cause damage to the tissues of the gut. In both cases the diarrhoea is beneficial for the pathogen because increase the chance that they will be spread to new hosts, because by-and-large they spread by the oral-faecal route.
The digestive tract is protected by the common mucosal immune system. There are locations throughout the digestive tract that act as monitoring points for potential antigens; together, these are known as GALT (Gut-Associated Lymphoid Tissue). These monitoring points include the tonsils and adenoids (forming a ring around the back of the throat called Waldeyer’s Ring). Further down the gut, you have Peyer’s Patches, a series of lymphoid tissue sites scattered throughout the gut. You also have the appendix and the isolated follicles of the intestine. All of these can detect antigens and trigger an adaptive immune response.
Thanks. What serves as the antigen? Is it the bacteria/virus itself, or some byproduct given off by the metabolism of the bacteria?
Anything that is not suppressed as a self antigen. It’s usually something structural, rather than a product of secondary metabolism, but - for instance - bacterial toxins can trigger an immune response. The problem, of course, is that for some of these toxins the fatal dose is lower than the immunogenic dose. Repeating polysaccharides are a potent antigen, though a poor immunogen - so they’ll trigger an adaptive response, but your immune system won’t ‘remember’ it, so it won’t be primed against a future infection that well. The reason they’re a potent antigen is that they are an important component of bacterial cell walls, and we have specific receptors that are triggered by them. In the case of viruses, it can be the actual capsid, it can be viral proteins expressed on the surface of an infected cell, it can even be the suppression of normal cellular ‘I’m okay’ signals by an invading virus (It’s quiet - TOO quiet.).
Also, you’re forgetting helminths. Lots of GI diseases are caused by eukaryotic parasites, rather than bacteria or viruses. The immune system can deal with those too, but not as effectively.