Final answer:
The most common types of lymphocytes in a blood smear are B cells and T cells, which play vital roles in the immune system, being involved in the adaptive immune response. B cells produce antibodies, while T cells have various functions that include regulating immune responses.
Step-by-step explanation:
The predominant type of lymphocyte found in a blood smear is either a B cell or a T cell. These cells are central to the adaptive immune response, with B cells maturing in the bone marrow and T cells maturing in the thymus. Both B and T cells initially develop from bone marrow and play crucial roles in the body's immune system. The human body has approximately 10¹² lymphocytes, and they make up about 20-30 percent of all leukocytes in the bloodstream.
B cells are responsible for producing antibodies which bind to specific antigens, while T cells differentiate into various types that help regulate the immune response or attack infected or malignant cells. Despite their critical roles in adaptive immunity, plasma cells differentiated from B cells also significantly contribute to humoral immunity by secreting soluble antibodies.