B Cells

B Cell Lymphocytes

B cell Lymphocyte
B cells are a type of white blood cell involved in immune response. They account for 10 percent of the body's lymphocytes. Credit: Science Photo Library - STEVE GSCHMEISSNER/Brand X Pictures/Getty Images

B Cells

B cells are white blood cells that protect the body against pathogens such as bacteria and viruses. Pathogens and foreign matter have associated molecular signals that identify them as antigens. B cells recognize these molecular signals and produce antibodies that are specific to the specific antigen. There are billions of B cells in the body. Unactivated B cells circulate in the blood until they come in contact with an antigen and become activated.

Once activated, B cells produce the antibodies needed to fight against infection. B cells are necessary for adaptive or specific immunity, which focuses on the destruction of foreign invaders that have gotten past the bodies initial defenses. Adaptive immune responses are highly specific and provide long lasting protection against the pathogens that illicit the response.

B Cells and Antibodies

B cells are a specific type of white blood cell called a lymphocyte. Other types of lymphocytes include T cells and natural killer cells. B cells develop from stem cells in bone marrow. They remain in the bone marrow until they become mature. Once they are fully developed, B cells are released into the blood where they travel to lymphatic organs. Mature B cells are capable of becoming activated and producing antibodies. Antibodies are specialized proteins that travel through the bloodstream and are found in bodily fluids.

Antibodies recognize specific antigens by identifying certain areas on the surface of the antigen known as antigenic determinants. Once the specific antigenic determinant is recognized, the antibody will bind to the determinant. This binding of the antibody to the antigen identifies the antigen as a target to be destroyed by other immune cells, such as cytotoxic T cells.

B Cell Activation

On the surface of a B cell is a B cell receptor (BCR) protein. The BCR enables B cells to capture and bind to an antigen. Once bound, the antigen is internalized and digested by the B cell and certain molecules from the antigen are attached to another protein called a class II MHC protein. This antigen-class II MHC protein complex is then presented on the surface of the B cell. Most B cells are activated with the help of other immune cells. When cells such as macrophages and dendritic cells engulf and digest pathogens, they capture and present antigenic information to T cells. The T cells multiply and some differentiate into helper T cells. When a helper T cell comes in contact with the antigen-class II MHC protein complex on the B cell's surface, the helper T cell sends signals that activate the B cell. Activated B cells proliferate and can either develop into cells called plasma cells or into other cells called memory cells.

Plasma B cells create antibodies that are specific to a specific antigen. The antibodies circulate in bodily fluids and blood serum until they bind to an antigen. Antibodies debilitate antigens until other immune cells can destroy them. It can take up to two weeks before plasma cells can generate enough antibodies to counteract a specific antigen.

Once the infection is under control, antibody production decreases. Some activated B cells form memory cells. Memory B cells enable the immune system to recognize antigens that the body has previously encountered. If the same type of antigen enters the body again, memory B cells direct a secondary immune response in which antibodies are produced more quickly and for a longer period of time. Memory cells are stored in the lymph nodes and spleen and can remain in the body for the life of an individual. If enough memory cells are produced while encountering an infection, these cells can provide life-long immunity against certain diseases.

Sources:

  • Immune Cells and Their Products. NIAID National Institutes of Health. Updated 2008 October 02. Available from: http://www.niaid.nih.gov/topics/immunesystem/immunecells/Pages/default.aspx
  • Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Helper T Cells and Lymphocyte Activation. Available from: http://www.ncbi.nlm.nih.gov/books/NBK26827/