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.
The immune system can mount a variety of responses to attack specific invader organisms. One of these responses is coordinated by T-helper cells (also known as T cells, T4 cells, or CD4 cells), which act as a kind of orchestra conductor. The T-helper cells tell other cells what to do when this response is triggered.