Abstract
In this chapter, we will present that T cells constitute up to 30% of all circulating leukocytes and are a major component of the adaptive immune system. In this context, we will describe that T cell immunity is, like B cell immunity, highly specific to antigens. The TCR is the specific antigen receptor of T cells and recognizes exclusively peptides displayed on MHC proteins. Modulated by the activity of accessory proteins, antigen-bound TCR initiates a signal transduction cascade that activates the same set of transcription factors as BCR signaling. Activation of TCR signaling results in the expansion of the respective T cell clone and the transformation to effector cells that contribute to the neutralization of microbes. We will learn that based on different type of antigen and signal exposure, CD4 + T cells differentiate into T H 1, T H 2 and T H 17 cells. The different T H cells diverge in their functional profile of coordinating other cells of the immune system in their fight against microbes. Finally, we will understand that cytotoxic T cells directly attack viral-infected host cells and can kill them by inducing apoptosis.