11.5B: Development of the Dual Lymphocyte System
- Examine dual lymphocyte development
The cells of the adaptive immune system are a type of leukocyte, called a lymphocyte. The human body has about 2 trillion lymphocytes, constituting 20-40% of white blood cells (WBCs); their total mass is about the same as the brain or liver. The peripheral blood contains 20–50% of circulating lymphocytes; the rest move within the lymphatic system. B cells and T cells are the major types of lymphocytes.
B Cell and T Cell Differentiation
Mammalian stem cells differentiate into several kinds of blood cell within the bone marrow. This process is called haematopoiesis. During this process, all lymphocytes originate from a common lymphoid progenitor before differentiating into their distinct lymphocyte types. The differentiation of lymphocytes into distinguishable types follows various pathways in a hierarchical fashion as well as in a more plastic fashion. The formation of lymphocytes is known as lymphopoiesis. B cells mature into B lymphocytes in the bone marrow, while T cells migrate to, and mature in, a distinct organ called the thymus. Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs (e.g. the spleen and lymph nodes) where they survey for invading pathogens and/or tumor cells.
Further Differentiation
The lymphocytes involved in adaptive immunity (i.e. B and T cells) differentiate further after exposure to an antigen; they form effector and memory lymphocytes. Effector lymphocytes function to eliminate the antigen, either by releasing antibodies (in the case of B cells), cytotoxic granules (cytotoxic T cells) or by signaling to other cells of the immune system (helper T cells). Memory cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure.
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Key Points
- B cells and T cells are the major types of lymphocytes.
- B cells mature into B lymphocytes in the bone marrow, while T cells migrate to, and mature in, a distinct organ called the thymus.
- Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs (e.g. the spleen and lymph nodes) where they survey for invading pathogens and/or tumor cells.
- The lymphocytes involved in adaptive immunity (i.e. B and T cells) differentiate further after exposure to an antigen to form effector and memory lymphocytes.
Key Terms
- lymphocyte : A type of white blood cell or leukocyte that is divided into two principal groups and a null group: B-lymphocytes, which produce antibodies in the humoral immune response, T-lymphocytes, which participate in the cell-mediated immune response, and the null group, which contains natural killer cells, cytotoxic cells that participate in the innate immune response.
- leukocyte : A white blood cell.
- haematopoiesis : Hematopoeisis is the formation of blood cellular components from a common progenitor stem cell.