Mus musculus (house mouse)
Antibodies (immunoglobulins) are the body's response to intruders from the outside, like bacteria, chemicals or viruses. As there are many different types of such intruders, called antigens, there are equally many different types of antibodies. Vaccines can be made by repeatedly injecting animals, often mice, with antigen and harvesting the produced antibodies. These antibodies usually vary slightly. In order to produce more specific antibodies, one can combine antibody-forming cells with tumor cells. This construction is called hybridoma and by growing such hybridomas in cell cultures, one can produce large amounts of identical antibodies. Antibodies created in this fashion are called monoclonal antibodies. The structure seen here is an example of such a monoclonal antibody.
The immunoglobulins are attached to the surface of a certain type of white blood cells, called B-cells, with the CH2:CH2/CH3:CH3 part of the molecule. When the antigen binds to the antibody molecule, changes in structure cause the B-cells to divide and create many more B-cells. The B-cells then release the antibodies in soluble form into their surroundings, producing large quantities of antibodies to attach to the intruding compound. This then is a signal for other cells and proteins in the body, to destroy the intruder marked by the antibodies. Sometimes the body's immune system may overreact by releasing excessive amount of histamine into the body. Such an overreaction is called an allergy. Allergies can be mild and cause minor inconvenience. Some however can be life-threatening and in the worst case cause death.
Antibodies are shaped like the letter Y. They consist of two types of protein chains, the light chains and the heavy chains. Each chain is formed by several regions (domains), variable (V) and constant (C) domains. The light chains each contain one V- and one C-domain and the heavy chains each contain one V- and three C-domains. These domains interact with each other in pairs. The bottom of the letter Y is made from pairs of heavy chain C-domains: CH2a:CH2b and CH3a:Ch3b. The arms of the letter Y are CLa:CH1a and VLa:VHa on one side and CLb:CH1b and VLb:VLHb on the other side. The connection between the CH1 and the CH2 domains is flexible and is called the hinge region. Attached to the molecule are also several sugar chains. At the top of the arms of the letter Y, in the VL:VH pairs, are the so-called hyper-variable loops. These are parts of the protein which recognize and tightly bind to features of the antigen. The interaction of these loops with the antigen molecule is very tight and specific. For each possible antigen the body forms a corresponding antibody.
Protein Data Bank (PDB)