Mononuclear cells refer to blood cells that have a single, round nucleus, such as lymphocytes and monocytes. When isolated from circulating blood, they are called peripheral blood mononuclear cells (PBMC), but other sources exist, such as the umbilical cord, spleen, and bone marrow.
A lymphocyte is any of 3 types of white blood cell in a vertebrate's immune system. All 3 are agranulocytes. They include natural killer cells (NK cells) (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic adaptive immunity), and B cells (for humoral, antibody-driven adaptive immunity). They are the main type of cell found in lymph, which prompted the name lymphocyte.