Why can b blood donate to AB blood??

Type B contains the B antigen.

This type is relatively rare (second only to AB). It contains the B antigen on its red blood cells, and the A antibody (to attack A antigens) in its plasma

Explanation:

it would lead to upper respiratory infections and pneumonia.

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands