B cells and antibodies

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What Are Antibodies?
Antibodies, also known as immunoglobulins, are Y-shaped proteins that are produced by the immune system to help stop intruders from harming the body. When an intruder enters the body, the immune system springs into action. These invaders, which are called antigens, can be viruses, bacteria, or other chemicals. When an antigen is found in the body, the immune system will create antibodies to mark the antigen for the body to destroy.

When a virus enters a human body, it will meet a type of cell called B cells. The B cells are responsible for creating antibodies to match the antigen. Remember, each type of antibody matches to only one antigen. After the B cells have created their antibodies, the antibodies stick to the virus, marking it for the next round of attack. T cells are then ordered to attack the antigen that the antibodies have marked for it.

After the antigen has been destroyed, the cleanup crew comes along. A wave of phagocytes, large cells that can consume foreign matter, eats the remains of the infection.

• B cells in the lymph nodes, spleen or other lymphatic tissue are responsible for antibody-mediated immunity.
• B cells differentiate into antibody secreting plasma cells.

B cell receptors
• Each B cell has a unique receptor similar to antibodies released from the B cell's daughter plasma cell.
• Genetic recombination is able to produce millions of different variations of receptors.
• Each B cell receptor binds to a specific unprocessed antigen.
• However, T cell costimulation is more commonly needed to activate B cells.

B cell activation
• In the primary method of activation the B cell begins by incorporating and digesting the antigen.
• Antigenic fragments and MHC-II (self identification) molecules are combined.
• Vesicles undergo exocytosis and antigen-MHC-II complexes are inserted into membrane.
• A Helper T cell with the appropriate receptor then binds to the presented antigen.
• The binding of the Helper T cell with the B cell triggers the release of Interleukins, which causes activation.

B cell proliferation and differentiation
• An activated B cell divides, replicating itself (clonal selection).
▪ Most of the replicated cells differentiate into plasma cells.
▪ The plasma cells release antibodies.
▪ These antibodies are able to bind to the same antigen as the receptor on the B cell that was originally activated.
• Some B cell offspring become B memory cells.
• These cells "remember" this antigen and respond rapidly when exposed to it in the future.

Actions of antibodies
• After activation, B cells differentiate to plasma cells which produce antibodies.
• Antibodies resist disease in several ways.

• Neutralization of viruses and toxins.
• Immobilization of bacteria.
• Agglutination and precipitation of antigen.
• Activation of complement.
• Enhancement of phagocytosis.