Cell Mediated Immune System (CMIS) is the branch of the adaptive immune system that protects the body against intracellular pathogens such as viruses, certain bacteria, fungi, parasites, and abnormal cells including cancer cells. Unlike humoral immunity, which is mediated by antibodies produced by B lymphocytes, CMIS primarily depends on T lymphocytes, macrophages, natural killer (NK) cells, antigen-presenting cells (APCs), and cytokines. It is particularly effective in eliminating infected host cells and providing long-term immunological memory. Cell-mediated immunity plays a vital role in organ transplant rejection, delayed hypersensitivity reactions, tumor surveillance, and defense against microorganisms that survive within host cells. The major cells involved in CMIS are T lymphocytes, which mature in the thymus. These include helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells. Antigen-presenting cells such as dendritic cells and macrophages process foreign antigens and present them to T cells through Major Histocompatibility Complex (MHC) molecules. Cytokines released by activated immune cells regulate and coordinate the immune response. Natural killer cells also contribute by destroying infected or malignant cells without prior sensitization. The mode of action of CMIS begins when a pathogen enters the body and infects host cells. Dendritic cells and macrophages engulf the pathogen through phagocytosis and process its proteins into