Regulatory T cells. Scanning electron micrograph (SEM) of regulatory T cells (red) interacting with antigen-presenting cells (blue). T lymphocytes, or T cells, are a type of white blood cell and a component of the body's immune system. They recognise a specific site on the surface of a pathogen or foreign object (antigen), bind to it and produce antibodies or cells to eliminate it. Regulatory T cells can suppress responses by T cells to maintain homeostasis in the immune system.