B and T cells are significant parts of the adaptive immune response which is also antigen specific immune response. These are the only cells in the body that are able to recognize the epitopes of the antigen and can respond specifically to each antigenic epitope. The B cells are responsible for the production of antibodies; therefore, the humoral immunity depends on the B cells. Cell immunity depends on the T cells. B cells are also known as antigen presenting cells because they present antigens and secrete cytokines. Immune-mediated cell death is carried out by two major subtypes of T cells: CD8+ "killer T cells" and CD4+ "helper" T cells. The aim of this book is to present researches that have transformed this discipline and aided its advancement. It explores all the important aspects of B and T cells. A number of latest researches have been included in this book to keep the readers up-to-date with the global concepts in this area of study.