T-cell responses are crucial for the outcome of any infection. The type of effector T-cell reaction is determined by a complex interaction of antigen-presenting cells with naive T cells and involves genetic and environmental factors, including the type of antigen, cytokines, chemokines, co-stimulatory molecules, and signalling cascades. The decision for the immune response to go in a certain direction is based not on one signal alone, but rather on many different elements acting both synergistically and antagonistically, and through feedback loops leading to activation or inhibition of T cells. In the course of evolution different types of T cells have developed, such as T helper 1 (Th1) cells, which protect against intracellular bacteria; Th2 cells, which play a role against parasites; and Th17 cells, which face extracellular bacteria and fungi.