An immune reaction through hypersensitivity
The over-reaction of the immune system brings about pathological changes in the tissues of the body known as an allergic response or allergy. Antigens and haptens that provoke an allergic response are known as allergens.
Common hypersensitivity responses include the reaction to poison ivy, pollens (hay fever) and some man-made (anthropogenic) chemicals acting as haptens.
In the above diagram leukocytes (white blood cells) which can circulate between the blood, the interstitial fluid and back to the blood again, are passing accross the endothelial wall of a capillary blood vessel.
Leukocytes are necessary for the immune response to invasion of the body by micro-organisms, viruses and toxins or foreign tissue.
The required leukocytes are transported in the blood to the site of the infection where adhesion molecules (or chemo-attractants) produced in response to the infection, selectins and integrins, aid the emigration of the leukocytes into the interstitial fluid to counter the invasion at source.