Cellmediated

Cell-mediated immunity refers to the part of the immune system that defends the body primarily through T lymphocytes and other cellular interactions rather than through antibodies. It relies on antigen presentation by specialized cells and the recognition of infected or abnormal cells by T cells, leading to targeted destruction or inhibition of the threat.

Key components of cell-mediated immunity include T lymphocytes, especially CD8+ cytotoxic T cells, which can directly kill infected or malignant cells, and CD4+ helper T cells, which coordinate responses by signaling other immune cells. Antigen-presenting cells such as dendritic cells, macrophages, and B cells present peptide antigens on major histocompatibility complex (MHC) molecules to T cells. MHC class I presents to CD8+ T cells and MHC class II presents to CD4+ T cells. Cytotoxic T cells destroy target cells through mechanisms such as perforin and granzymes, Fas-FasL interactions, and cytokine release that activates macrophages.

The helper T cell response, particularly Th1 cells, activates macrophages and supports the development of cytotoxic responses and memory. Regulatory T cells help maintain tolerance and prevent autoimmunity. Memory T cells provide faster responses upon re-exposure to the same antigen.

Cell-mediated immunity is especially important against intracellular pathogens (viruses, some bacteria and fungi) and in tumor surveillance and graft rejection. It operates alongside humoral immunity, which is antibody-mediated. Clinically, defects in T-cell function can lead to increased susceptibility to infections, as seen in HIV/AIDS, and regulatory imbalances can contribute to autoimmune or inflammatory diseases.