Tcellmediated

T cell–mediated immunity refers to the portion of the adaptive immune response driven by T lymphocytes. It is primarily responsible for recognizing antigenic peptides presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells and coordinating cellular defenses against intracellular pathogens, malignant cells, and allografts.

Activation begins when naive T cells encounter their cognate peptide-MHC complex on dendritic cells, with additional co-stimulatory signals. This triggers clonal expansion and differentiation into effector CD4+ helper T cells and CD8+ cytotoxic T lymphocytes, followed by formation of long-lived memory T cells. The balance of cytokines guides differentiation; Th1 responses favor intracellular pathogens and macrophage activation, while Th2 responses tend toward humoral help.

CD8+ cytotoxic T cells kill target cells that display foreign or abnormal peptides via perforin and granzymes, and through Fas-FasL–induced apoptosis. They can recognize infected cells presenting peptides on MHC class I. Helper CD4+ T cells support cytotoxic responses and macrophage activation by secreting cytokines such as IFN-γ and IL-2. Regulatory T cells (CD4+CD25+ FOXP3+) temper responses to maintain tolerance and prevent excessive tissue damage.

Memory T cells provide rapid responses upon re-exposure. T cell–mediated mechanisms feature prominently in delayed-type hypersensitivity reactions, transplantation rejection, and tumor immune surveillance, as well as defense against intracellular pathogens that evade antibody neutralization.

Compared with humoral immunity, which primarily uses antibodies produced by B cells, T cell–mediated immunity acts directly on infected or abnormal cells and coordinating immune cell functions. Dysregulation can contribute to autoimmune diseases such as type 1 diabetes, multiple sclerosis, and rheumatoid arthritis. Assessment of T cell responses can involve assays like ELISpot or skin tests measuring delayed-type hypersensitivity.