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.
There are four types of T cells, each with a special purpose. Helper T cells detect infection and get the other cells of the immune system ready for battle. Helper T cells also tell B cells to produce antibodies, highly specialized proteins that help defend the body against infection. Cytotoxic, or killer, T cells destroy infected cells. Regulatory T cells tell the immune system when the battle with the pathogens is over and to stop fighting. Memory T cells remember how to defeat an infection and can respond rapidly if the same infection reoccurs.