Why is it important to understand the hla phenotypes of the individuals in this case?

The 'case' that is mentioned this question is not defined here.

I will put up a general understanding of the importance of understanding the HLA phenotypes.

The HLA stands for Human Leucocyte Antigen which is responsible of the regulation of immune system in the body. They have many different alleles that are responsible to fine tune the adaptive immune system of the body.

These antigens are the major cause of organ transplant rejections. The reason behind this is that the phenotypes of the recipients and the donors have to be necessarily matched failing which the antigens in the immune complex with reject the new organ taking it for an foreign object. Hence before transplantation, there is a detailed study of the match of the HLA phenotypes.

a physical change is being made.

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.