The main function of our immune system is to fight infectious diseases caused by harmful (pathogenic) strains of bacteria, viruses, fungi and protozoa. If infected, specific proteins (called antigens) on the surface of the pathogen trigger an immune response, involving secretion of antibodies that match the antigen and neutralize and destroy the pathogen. The problem is that foods, pollen etc also have antigenic properties – even though they are harmless. To deal with this, the immune system has a “regulatory arm” which is programmed to recognize non-harmful agents and ensure they are not attacked. 

To do this the immune system, like a computer, must be programmed with data enabling it to distinguish antigens of harmful microbes from antigens that need to be ignored. Programming occurs particularly in early life, by ongoing exposure to Old Friends microbes. If we are not sufficiently ”programmed”, the immune system may fail to recognise food or pollen as harmless and attack it. It is this “attack” which triggers the allergic symptoms that we experience.