Abstract: Innate immunity and adaptive immunity form the two main components of human immune system. Unlike the innate immunity, adaptive immune response is specific and more complex which may be cell-mediated or humoral-mediated. T lymphocytes, the effectors of cell-mediated immunity (delayed hypersensitivity) are immune cells derived from bone marrow and develop in the thymus. The diversity of the T cell receptor enables recognition of wide variety of specific antigens expressed by antigen presenting cells (APC) in the Major Histocompatibility (MHC) context. Following complete activation, T cells differentiate into any of the subtypes depending on the cytokine milieu. Plasticity among T helper subsets have also been noted. Knowledge of how the immune-inflammatory mechanisms are regulated is essential for understanding the pathogenesis of inflammatory diseases like periodontitis where the complex interplay between microbial flora and the host immune mechanisms govern the balance between tissue homeostasis and disease progression. This review highlights on the processes of development, subsequent activation of T cells, and differentiation into various subtypes, briefly summarizing their role in periodontal disease.