The human immune system is an elegant system of intricately inter-related & inter-dependent players (T cells, B cells, etc, etc.). There is a degree of overlap in function so that, no matter the invader or means of invasion, the human immune system has an (or several) answer(s) for whatever problem the latest pathogen has to offer. What is it about T cells, then, that their deficiency can lead to so many problems with immune response to various pathogens? Use specific examples in your discussion.