This is a great overview of methylation, histamine intolerance, MAST cell disorders and gene variants (like MTHFR) From a woman whose 5yo has been really sick. via the Low Histamine Chef:

Our goal in helping our little guy was to lower the histamine we were putting IN his body,reduce the amount of the chemicals that were competing for the same pathway out, and lighten up the functional bottleneck that was backing up histamine in his body. Part of the reason this was happening was his genetic hiccups in the methylation cycle inhibit some of the enzymes that act as the ‘bouncers’. The liver secretes these molecules in order to escort the chemicals you don’t want in your body anymore to the door…or the toilet, as the case may be.

Source: Histamine, Mast Cells and Methylation: A Mother Battles MCAD | THE LOW HISTAMINE CHEF