Diagrammatic representation of T-cell differentiation and function. Lymphoid progenitors enter the thymus where precursor T cells develop into varied types of naive T cells. The cells of the innate immune system include NK cells, γδ T cells, and NK-like T cells. These cells constitute a primitive type of immune response that lacks both specificity and memory. In the adaptive immune system, αβ T cells leave the thymus, where, upon exposure to antigen, they may undergo blast transformation and develop further into CD4⁺ and CD8⁺ effector and memory T cells. T cells of the adaptive immune system are heterogeneous and functionally complex, and include naive, effector (regulatory and cytotoxic), and memory T cells. Another specific type of effector T cells is the follicular helper T-cell that is found in GCs (TFH). Upon antigenic stimulation, T-cell responses may occur independent of the GC, or in the context of a GC reaction. The lymphomas of the innate immune system are predominantly extranodal in presentation, mirroring the distribution of the functional components of this system.¹¹³  T-cell lymphomas of the adaptive immune system present primarily in adults, and are mainly nodal in origin.