What immune cell is primarily responsible for cellular immunity?

The CD8 T cell is primarily responsible for cellular immunity, which is a crucial aspect of the adaptive immune response. These cells, also known as cytotoxic T lymphocytes, play a key role in identifying and destroying infected or malignant cells. They achieve this by recognizing specific antigens presented by Major Histocompatibility Complex (MHC) class I molecules, which are expressed on nearly all nucleated cells.

Once activated by recognizing an antigen, CD8 T cells can directly kill target cells through the release of cytotoxic granules containing perforin and granzymes, which induce apoptosis in the targeted cells. This function is vital for controlling viral infections and for eliminating tumor cells, highlighting the importance of CD8 T cells in maintaining cellular immunity.

While other immune cells like B cells and CD4 T cells (helper T cells) have important roles in the immune response—B cells are responsible for humoral immunity and antibody production, and CD4 T cells mainly assist other immune cells in responding to pathogens—it's the CD8 T cell that is specifically tasked with directly combating intracellular pathogens and providing cellular-level immune defense.