There is a Dr. Rosemary Waring at Birmingham University who has researched gut immunity in the context of autism and ASD. She has discovered that a high percentage of autistic children have poor sulphation mechanisms. In real terms, this means that the mucins that line the gut, preventing pathogenic bacteria/ fungi from attacking the gut wall, are not smooth and slippery like they should be, but lumpy and clumpy and without the appropriate negative charge.
This means that the fungus Candida albicans (causes thrush) is not repelled and can grow more than normal, and can potentially create a leaky gut wall by "going hyphous" and penetrating it. Leaky gut has been linked with many auto-immune conditions as well as autism and ASD.
Children who have low sulphation potential may be more at risk from vaccines like the MMR; they might also be more susceptible to being intolerant of dairy and wheat (among other foods). They are likely to be at risk for other allergic/ autoimmune conditions such as eczema, asthma (for. e.g.).
There is no NHS test for sulphation potential. But if you come from a family with a high history of allergic/ autoimmune conditions, it is possible that the cause may be linked to poor sulphation, and the child might then be at higher risk of suffering a reaction to the MMR (or other vaccine)
This is a very short summary of one potential theory on how vaccines may contribute to ASD conditions. Dr. Rosemary Waring is a respected biochemistry researcher and her results make interesting reading.