A common problem for the purple leaf plum tree is blossom blight, in which first the flowers, then young leaves, turn brown and wither, according to Grow Fruit by Alan Buckingham.
Once activated, helper T cells start to divide and to produce proteins that activate B and T cells as well as other immune cells. The killer T cell is specialized in attacking cells of the body infected by viruses and sometimes also by bacteria.
Dendritic cell are immune system cells whose function is to identify cancer cells, and pathogens like viruses, bad bacteria and fungus, so the the immune system T cells can then kill them. If a cancer cell is not identified by a dendritic cell, the T cells won't know it needs to be killed.