Cell-mediated immunity represents an adaptive immune response that microbes inside host cells trigger. T lymphocytes mediate it, and unlike humoral immunity, only T cells, not antibodies, can transfer it. This response is essential for defending against intracellular infections and regulating various immune processes.
CD4+ helper T lymphocytes are capable of differentiating into several effector cell types, each specialized in combating different pathogens. One of the main subsets is the Th1 cells, which secrete IFN-γ and specialize in defending against intracellular microbes by activating macrophages to kill phagocytosed pathogens. This process releases reactive species and enzymes, triggering inflammation that may also cause tissue damage.
These cells promote IgE production by B cells, facilitating mast cell degranulation and inflammation. Additionally, IL-5 activates eosinophils, which destroy helminths but can also damage host tissues. A third subset of CD4+ cells is Th17 cells, which play a crucial role in defending against extracellular fungi and bacteria. These cells promote neutrophil-rich inflammation and can also contribute to the tissue damage observed in autoimmune diseases.
Cytokines from antigen-presenting cells (APCs), T cells, and other immune cells regulate the differentiation of naïve T cells into effector subsets. Transcription factors control this process by activating specific cytokine genes, ensuring stable differentiation toward a particular T cell type. Cytokines from each subset promote its development and limit other populations, boosting immune response polarization.
Another relevant group in cell-mediated immunity is γδ T cells, which have receptors with limited diversity and do not require antigen presentation through the major histocompatibility complex (MHC). These cells can recognize a variety of antigens, produce cytokines, and participate in host defense and inflammatory diseases.
In summary, cell-mediated immunity is crucial for defending against various pathogens, including intracellular ones, helminths, and fungi. T cells regulate inflammation and differentiate into subsets, balancing infection protection and preventing autoimmune diseases or tissue damage.