46% to 84% of autistic individuals have GI-related problems like reflux, diarrhea, constipation, inflammatory bowel disease, and food allergies. It has been observed that the makeup of gut bacteria in autistic people is different than that of neurotypical individuals which has raised the question of influence of gut bacteria on ASD development via inducing an inflammatory state. Listed below are some research findings on the influence of gut bacteria and abnormal immune responses on brain development:

Some studies on rodents have shown gut bacteria influencing emotional functions and neurotransmitter balance in the brain, both of which are impacted in ASD.
The immune system is thought to be the intermediary that modulates the influence of gut bacteria on the brain. Some ASD individuals have a dysfunctional immune system with higher numbers of some types of immune cells, biochemical messengers and modulators, and autoimmune antibodies. Increased inflammatory biomarkers correlate with increased severity of ASD symptoms and there is some evidence to support a state of chronic brain inflammation in ASD.
More pronounced inflammatory responses to bacteria were found in ASD individuals with an abnormal gut microbiota. Additionally, immunoglobulin A antibodies that are central to gut immunity were also found in elevated levels in ASD populations. Some of these antibodies may attack proteins that support myelination of the brain, a process that is important for robust transmission of neural signal in many nerves.

Activation of the maternal immune system during pregnancy (by gut bacteria, bacterial toxins, an infection, or non-infectious causes) and gut bacteria in the mother that induce increased levels of Th17, a pro-inflammatory immune cell, have been associated with an increased risk of autism. Some maternal IgG antibodies that cross the placenta to provide passive immunity to the fetus can also attack the fetal brain.
It is proposed that inflammation within the brain promoted by inflammatory responses to harmful gut microbiome impacts brain development.[170]
Pro-inflammatory cytokines IFN-γ, IFN-α, TNF-α, IL-6 and IL-17 have been shown to promote autistic behaviors in animal models. Giving anti-IL-6 and anti-IL-17 along with IL-6 and IL-17, respectively, have been shown to negate this effect in the same animal models.
Some gut proteins and microbial products can cross the blood–brain barrier and activate mast cells in the brain. Mast cells release pro-inflammatory factors and histamine which further increase blood–brain barrier permeability and help set up a cycle of chronic inflammation.