B cells, also known as B lymphocytes, are a type of white blood cell that operates within the adaptive immune system, which provides a targeted defense against specific threats. The job of B cells is to protect the body from foreign substances, such as bacteria and viruses. Circulating through the blood and lymphatic system, their functions are a component of humoral immunity, which focuses on threats found in the body’s fluids.
B Cell Development and Activation
Every B cell begins its journey from hematopoietic stem cells in the bone marrow, where they develop and mature. During this development, each B cell produces a unique protein on its surface called a B cell receptor (BCR). This receptor is designed to bind to one specific molecular structure, an antigen, which is part of a pathogen.
Once a B cell leaves the bone marrow, it travels to secondary lymphoid organs like the spleen and lymph nodes, where it may encounter its specific antigen. Activation begins when the B cell’s receptor binds to its matching antigen. For full activation, this initial binding is followed by help from a T helper cell, which recognizes the same antigen and provides signals that stimulate the B cell.
Antibody Production and Action
Upon activation, the primary mission for most B cells is to transform into plasma cells. These plasma cells are specialized factories dedicated to producing proteins called antibodies. A single plasma cell can secrete thousands of antibody molecules per second, which are custom-built to target the specific antigen that triggered the activation.
Antibodies employ several methods to combat pathogens. One action is neutralization, where antibodies bind to a virus or bacterium, blocking it from infecting the body’s cells. Another method is opsonization, which involves antibodies coating a pathogen. This coating acts as a flag, making the invader easier for phagocytic cells, like macrophages, to identify and destroy.
A third function is complement activation. When antibodies bind to a pathogen’s surface, they can initiate a cascade involving blood proteins known as the complement system. This activation leads to protein interactions that can directly destroy the pathogen by forming pores in its cellular membrane, causing it to die.
Formation of Immunological Memory
While many activated B cells become short-lived plasma cells, a subset differentiates into memory B cells. These are long-lived cells that provide the basis for lasting immunity. Memory B cells circulate throughout the body for years, sometimes for a lifetime, holding a record of the pathogen they were designed to fight.
The function of memory B cells is to provide a swift defense if the same pathogen is encountered again. Upon re-exposure, memory B cells activate much more quickly than naive B cells did during the initial infection. They rapidly proliferate and differentiate into plasma cells, producing antibodies to neutralize the threat before it can cause illness. This rapid secondary response is the principle behind vaccines, which introduce a harmless version of a pathogen to generate memory cells.
Role in Antigen Presentation
Beyond producing antibodies, B cells perform another communication function as antigen-presenting cells (APCs), a role shared with cells like macrophages and dendritic cells. This process begins when a B cell uses its receptor to bind to and internalize a foreign antigen.
Once inside the B cell, the antigen is broken down into smaller fragments. The B cell then displays these fragments on its surface using major histocompatibility complex (MHC) class II molecules. By presenting this antigen piece to T helper cells, the B cell helps activate T cells that recognize the same antigen, which coordinates the adaptive immune response.
B Cells in Disease
When the B cell system is disrupted, it can lead to disease. In cases of immunodeficiency, the body may lack properly functioning B cells, such as in X-linked agammaglobulinemia (XLA). This genetic disorder results in a severe shortage of B cells, meaning the body cannot produce antibodies and is highly susceptible to recurrent bacterial infections.
Conversely, problems arise when B cells mistakenly identify the body’s own tissues as foreign, leading to autoimmune diseases. B cells produce autoantibodies that target and attack healthy cells. In systemic lupus erythematosus, B cells generate autoantibodies against the body’s DNA and proteins, causing widespread inflammation. In rheumatoid arthritis, autoantibodies attack joint tissues, leading to chronic inflammation and bone erosion.