When a patient has a type of thalassemia, there is often an excess production or accumulation of globin chains produced by genes that are not effected by the thalassemia deletion. This is a compensation mechanism that the body utilizes to maintain hemoglobin production (which requires globin chains).
In alpha thalassemia, the body can produce excess gamma chains as a compensatory mechanism. This can lead to the production of gamma chain tetramers (hemoglobin Bart's) in the unborn child and as beta chain tetramers (hemoglobin H) in adults.
This subsequent tetramer accumulation in response to thalassemia often leads to red blood cell damage and hemolytic anemia.