Sometimes under certain environmental conditions such as high altitudes, low oxygen and increased atmospheric pressure, those with sickle cell trait experience sickle cell crisis.
The sickle cell trait is related to lower concentration of S hemoglobin. The sinptoms appear when the condition are extreme such as you have mentioned. The higher concentration of HbA compared with homozigous SS make then more stable.However when the condition is extreme the low percentage of hemoglobin S may fold causing deformation of RBC. Is quite impossible to see sickle cell in smear from HBAS
In sickle cell trait the development of SCD symptoms is very rare but possible. Is always suggested to control diagnosis in order to exclude coinheritance of other red cell defect.
There is 40% sickle i.e. mutant Hb in each RBC. When the sickle cell carrier go to high altitude or do strenuous exercise, his remaining normal Hb can not cope up with this and the carriers also (most of the sport persons) under go SCD like pains.
As other answers point out, this is due to the relative presence of the HbS hemoglobin, that is not capable of producing symptoms. However, this is not an absolute protection, because some situations, like extreme exercise or pregnancy and childbirth can cause symptoms.
Dear it happens because of high level of normal adult Haemoglobin.
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