Final answer:
CTLA-4 and PD-1 are expressed in T cells and have important roles in regulating the immune response. CTLA-4 inhibits T cell activation, while PD-1 regulates T cell activity through immune checkpoint signaling.
Step-by-step explanation:
CTLA-4 (cytotoxic T lymphocyte antigen-4) and PD-1 (programmed cell death antigen-1) are expressed on T cells and have important functions in regulating the immune response.
CTLA-4 is mainly expressed in activated T cells and regulatory T cells. It functions as an inhibitory receptor that downregulates immune responses by competing with the co-stimulatory molecule CD28 for binding to CD80/CD86 on antigen-presenting cells (APCs). By inhibiting T cell activation, CTLA-4 helps to prevent excessive immune responses and maintain immune homeostasis.
PD-1 is expressed in activated T cells, B cells, natural killer (NK) cells, and macrophages. It functions as an immune checkpoint receptor that regulates T cell activity. When PD-1 binds to its ligands, PD-L1 and PD-L2, on APCs or other cells, it inhibits T-cell activation and promotes immune tolerance. This mechanism helps to prevent autoimmune reactions and immune-mediated tissue damage.