B cell (B lymphocyte)- Definition, Types, Development, Applications


B cells, also known as B lymphocytes, are a type of white blood cells that play a vital role in the adaptive immune system. They are responsible for producing antibodies that can recognize and neutralize foreign invaders such as bacteria, viruses, and toxins. B cells are also involved in presenting antigens to helper T cells, which can activate other immune cells and enhance the immune response. B cells are named after the bursa of Fabricius, an organ in birds where they were first discovered. In mammals, B cells develop in the bone marrow and then migrate to various lymphoid organs such as the spleen, lymph nodes, and mucosal tissues.

The immune system consists of two main branches: the innate immune system and the adaptive immune system. The innate immune system is the first line of defense against pathogens and provides a rapid and nonspecific response. It includes physical barriers such as skin and mucous membranes, as well as cellular components such as macrophages, neutrophils, natural killer cells, and dendritic cells. The adaptive immune system is the second line of defense and provides a specific and long-lasting response. It involves two types of lymphocytes: B cells and T cells. B cells mediate humoral immunity, which is based on the production of antibodies that circulate in the blood and lymph. T cells mediate cellular immunity, which is based on the direct killing of infected or abnormal cells by cytotoxic T cells or the regulation of other immune cells by helper T cells.

B cells and T cells have different mechanisms of recognizing antigens. Antigens are molecules that can elicit an immune response by binding to specific receptors on lymphocytes. B cells have receptors called B cell receptors (BCRs) that are composed of membrane-bound immunoglobulins (Igs). Each B cell has a unique BCR that can bind to a specific antigen. T cells have receptors called T cell receptors (TCRs) that are composed of two different chains: alpha and beta. Each T cell has a unique TCR that can bind to a specific antigen only when it is presented by a major histocompatibility complex (MHC) molecule on another cell. MHC molecules are proteins that display fragments of antigens on the surface of antigen-presenting cells (APCs) such as dendritic cells, macrophages, or B cells.

When B cells encounter an antigen that matches their BCR, they become activated and undergo several processes such as proliferation, differentiation, class switching, affinity maturation, and memory formation. These processes enable B cells to produce large amounts of antibodies with high specificity and diversity, as well as to generate long-lived memory B cells that can provide protection against future infections by the same antigen. The antibodies produced by B cells can perform various functions such as neutralizing pathogens or toxins, opsonizing pathogens for phagocytosis by macrophages or neutrophils, activating complement system for lysis of pathogens or infected cells, or facilitating antibody-dependent cellular cytotoxicity (ADCC) by natural killer cells or eosinophils.

B cells are essential for the immune system as they can protect the body from various infections and diseases. However, B cells can also cause problems when they malfunction or become dysregulated. For example, B cells can produce autoantibodies that attack self-tissues and cause autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, or multiple sclerosis. B cells can also undergo malignant transformation and give rise to cancers such as leukemia or lymphoma. Therefore, understanding the biology and function of B cells is important for developing new strategies for immunotherapy and immunomodulation.