Tsejt

Tsejt, also known as a T cell, is a type of lymphocyte central to the adaptive immune system. They originate from hematopoietic stem cells and mature in the thymus, where their T cell receptor (TCR) repertoire is shaped by positive and negative selection. Mature Tsejtek express the TCR–CD3 complex and participate in antigen-specific immune responses.

Activation occurs when a naive Tsejt recognizes a peptide antigen presented by MHC molecules on antigen-presenting cells. CD4+ Tsejtek (helper T cells) respond to peptides on MHC class II and secrete cytokines that help activate B cells, macrophages, and other T cells. CD8+ Tsejtek (cytotoxic T cells) recognize peptides on MHC class I and can kill infected or malignant cells by releasing cytotoxic granules such as perforin and granzymes or through death receptors like Fas/FasL. Co-stimulatory signals, such as CD28–B7 interactions, are typically required for full activation.

Diversification yields regulatory T cells (Tregs; often FoxP3+) that suppress immune responses to maintain tolerance and prevent autoimmunity. Other subsets include Th1, Th2, Th17, and T follicular helper (Tfh) cells, each with distinct cytokine profiles that tailor the immune response and assist B cells in germinal centers. Memory T cells persist after infection or vaccination to provide faster responses upon re-exposure. Gamma-delta T cells and MAIT cells are less conventional, with innate-like features.

Clinical relevance includes immunodeficiencies affecting T cells, autoimmune diseases, and cancer immunotherapies that harness T cells, such as CAR-T cell therapy and checkpoint inhibitors used to enhance T cell function.