T cells refer to a type of white blood cells found in the immune system. They play an important role in the adaptive immune response. They respond to the antigen-presenting cells of the innate immune system and induce immune responses by releasing cytokines. T cells are found in the bone marrow and they migrate to the thymus, where they mature. These cells have the ability to differentiate into different subtypes of T cells including T helper 1 (TH1) cells, a T helper 2 (TH2) cells, or a memory helper T cells. Apart from helper T cells and memory cells, regulatory T cells and cytotoxic T cells are also found in the immune system. These classes are distinguished by their functional roles in the adaptive immunity, expression of certain surface molecules, and the mode of activation. The main functions of T cells include generating cytokines, killing infected host cells directly, controlling the immune response, and activating other immune cells. This book provides comprehensive insights into the biology of T cells. It presents researches and studies performed by experts across the globe. The extensive content of this book provides the readers with a thorough understanding of the subject.