Parasites they grow in the host shelter and feed of it
The T cell is a lymphocyte or type of white blood cell. They play a role in the immunity of the body. There are different varieties of T cells and they are differentiated by the receptors that are on the surface of the cells. Some cells are natural killers and some are B cells.
Some molecule can pass through but others cannot because they are too large to pass through. This is also called selective permeability.