Hi CinC, the "granules" in these cells have extreme biologic activity on body chemistry.
Some of these things are serotonin, and histamine, as well as acetlylcholine [and many others].
These are messenger molecules which can act upon glands, such as those in and near the stomach, causing regulation of secretions. In some cases, there may over secretion of stomach acid, and in others, decreases for example.
Stomach acid is important for killing microbes, and maintaining stable PH in the bowel system.
As for microbes, the balance of bacteria create PH conditions, and the body will respond to any changes by regulation processes. There may be fluctuations, which cause symptoms, but generally the PH levels are fairly stable.
The microbial environment can be altered, in the upper bowel due to mast cell activity [caused by parasite\pathogen] and lactobacteria diminish. Compensation for digestion will turn to lower bowel bacteria, which are generally falcultative aenarobic types like e-coli. Fungal activity tends to be higher as well. It is apparent, that subtle changes in PH make big differences, but aforementioned bacteria generally like neutral to slightly alkaline environment. Shroom