T cells are amazing white blood cells that play a crucial role in cell-mediated immunity. They can be divided into several categories - notably helper, cytotoxic, memory, regulatory, and natural killer (NK) - and their primary role within the immune system is to search out and destroy both invading pathogens and abnormal cells. Helper T cells also help to regulate the immune response.
The image above features a cytotoxic T cell, which destroy abnormal cells (most notably virally infected cells and tumour cells) and are also implicated in organ transplant rejection. Dendritic cells, pictured cyan blue in the photo, constitutively express high levels of both class I and class II MCH molecules - the type of receptor that cytotoxic T cells recognise - making them extraordinary potent activators capable of stimulating even “naive” T cells that have never encountered an antigen.