Immunodiagnostics

B lymphocytes, also known as B cells, are important components of the adaptive immune system. In the bone marrow, the hematopoietic stem cells go through a series of steps to become mature naive B cells. The cells migrate to secondary lymphoid organs for activation and further maturation. The process entails antigen stimulation, with or without the help of T cells. The T-cell–independent activation generates a short-lived immune response (via plasma cells), and this is seen with antigens such as bacterial lipopolysaccharides. T-cell–dependent activation, on the other hand, produces both plasma cells and memory cells. Activated B cells then proliferate in the germinal centers, but not all become effector B cells. Through somatic hypermutation, B cells undergo additional mechanisms to increase the affinity of the antibody to the antigen. Only those with high-affinity B-cell receptors subsequently advance for terminal differentiation. B cells then go through class switching (from IgM to another class of Ig) under the influence of cytokines. After class switching, the B cells become plasma cells (which produce antibodies) or memory cells (which mount a robust secondary immune response).

T cells, also called T lymphocytes, are important components of the adaptive immune system. Production starts from the hematopoietic stem cells in the bone marrow, from which T-cell progenitor cells arise. These cells migrate to the thymus for further maturation. A functional mature T cell develops from a step-by-step process creating a T-cell receptor (TCR) complex, selecting T cells with appropriate affinity to self-antigens associated with major histocompatibility molecules (positive selection), and expressing either CD4 or CD8. In this series, cells predisposed to autoimmunity undergo apoptosis (negative selection). When released from the thymus, the naive mature T cells travel to the secondary lymphoid organs for activation. Two signals, an antigen-specific binding of TCR and costimulation, are required to be activated (ready to mount an immune response). In the case of CD8+ T cells, additional cytokine stimulation is necessary. Depending on the cytokines they are exposed to during antigen stimulation, the undifferentiated mature T cell (Th0) develops into cells with different functions: CD4+ become T helper (Th) cells and CD8+ become cytotoxic, or cytolytic, cells. Th cells have other subtypes; the most characterized are Th1, Th2, Th17, follicular Th cells, and regulatory T cells. Other types include natural killer T cells and memory T cells. These mature differentiated T cells ensure effective surveillance and immediate response to pathogens, tumor cells, and foreign tissues and provide immunologic memory.