How does the digestive system work with the immune system?
To tell the truth digestive system,through out its whole length,lies in association with several commensal bacteria.As for example within our mouth we harbour different species of Gram positive and negative cocci like peptococcus,peptostreptococcus,streptococcus salivarius etc. With in gut we have Escherichia coli,Clostridium ramosum,Clostridium welchii etc.
They live in a commensal relationship so that both the host and bacteria can get the benifit.
But they are not so obident to the gut alone,and can invade whenever get any opportunity and cause several diseases like dental root canal resorption,or diarrhoea like symptoms.
Even you will get astonished to know that now a days scientist are indicating that bad gum hygiene can lead to Rheumatoid Arthritis due to molecular mimicry by these commensals!!!
But it's not that all the time we suffer from their attack,and it is because,along the gut different aggregation of lymphoid follicles are present in the lamina propia of mucosa which checks their growth and entry,so only when body immune system fails they get access to our body.
As for example we can name the Palatine tonsil,Nasopharyngeal tonsil(part of Waldeyer's ring).
In the small and large intestine the Payer's patches(with microfold cells or M cells i.e antigen presenting cells etc.
But it's not that all their efforts can block entry by all pathogens. Strictly speaking they can only fight against opportunistic pathogens only.
For prevention against pathogenic bacteria the major role is played by Gastric acid ,HCL which kills a lot of pathogenic bacteria like Vibrio,Salmonella etc.
Though Salmonella has got acid tolerance gene which can help it to escape from the effects of HCL if the patient is having underlying gastric acid disorders like chronic atrophic gastritis or is a chronic Proton Pump Inhibitor intaker.
Another important thing is Surface or Mucosal immunity,mediated by IgA.They have a major role in preventing against infection by Enterovirus(including Polio virus,Coxsackie etc.)So,children with impaired mucosal immunity were found to be more infected with polio and the Oral Polio Vaccine or OPV was targated to enhance this mucosal immunity.