In order to understand the mechanism of allergy injections we must first review the allergic response.
When pollen, dander, or another allergen is introduced into the body of an allergic person, the body may respond by developing allergic antibodies (lgE). As more and more allergens over time are introduced into the body, more allergic antibodies are made. Allergic antibodies attach to specialized allergy cells called mast cells. On future allergen exposure, the mast cell is signaled to release histamine, leukotriene and other allergy causing chemicals that attach to various parts of the body producing symptoms commonly seen with allergies like: sneezing, itchy watery eyes, runny nose, congestion, wheezing, cough, etc.
Allergy injections stimulate the body to make blocking antibodies. Blocking antibody, as the name implies, blocks the attachment of allergens to allergic antibodies on mast cells, thereby preventing the release of the allergy causing chemicals (histamine, leukotriene, etc.) The longer the stimulation and production of blocking antibody (i.e. the longer allergy injections continue) the more complete the blocking of the mast cells. Also, while allergy injections are increasing blocking antibodies, allergic antibodies are decreasing, thereby producing effective results by two mechanisms.