Google search would be much more reliable, but of what i know the parasite is unable to attach properly to sicle shaped blood cells.

The sickle-allele hemoglobin is "sickled" when not attached to oxygen. Infection of an RBC by malaria parasites deoxygenates the cell, sickles those hemoglobins (in heterozygotes, who have a decent percentage of sickle-cell hemoglobins in each cell), and the cell is processed as defective – that is, killed. I think by T cells, but I’ve forgotten that detail.

So homozygous "regular" hemoglobins puts one at greater risk from malaria, homozygous sickle-cell hemoglobins puts one at risk from sickle-cell (even regular deoxygenation can distort RBCs to the point that they won’t go through capillaries properly), and heterozygotes get some protection from both.

from what i know and learned, sickle cell causes erythrocyte to shape abnormally (like sickle). because Plasmodium (protozoans which cause malaria) attacks and live in erythrocyte in one of their life stages, unusual shape of cell stops Plasmodium from living there. sickle-shaped erythrocyte also has abnormal protein that is different from healthy cell, which inhibits Plasmodium from developing there.
sickle cell is caused by recessive alleles, that’s why a heterozygote person still has normal red blood cell, but is better protected against malaria than a homozygote dominant person.
it is more common to find heterozygotes nowadays because of a mechanism called "stabilizing selection". it cuts off the homozygotes part and preserves heterozygote individuals because perhaps heterozygotes are more favorable to environment. that is part of natural selection and evolution.
however, homozygote recessive person cannot develop malaria at all but on the other hand, get sickle cell disease.

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