We all react to polutants in some way but some of us have a hyper-response. This can really interfere with life because it hits us and stops our activities.Anything that comes into contact with our body can generate an allergic response <allergen>
Skin and breathing problems can be the most interfering types of allegies.
Allergies are part of our immune system.
Mammals have a unique feature of the immune system of histamine. This took millions of years to establish and is an immune system against single celled parasites. Originally called a single celled animal because they can move around, motility. We know this well as malaria and yellow fever from infected mosquito saliva entering our body.
The immune system is often referred to as IgE - Immuno-globulin E, part of the white blood cell complex.