Immune System

Antibodies are made when a B cell specific for the invading antigen is stimulated to divide by the binding of an APC to a helper T cell. The dividing B cells, called plasma cells, secrete proteins called antibodies. Antibodies are composed of a special type of protein called immunoglobin (Ig). An antibody molecule is Y-shaped and consists of two light chains joined to two heavy chains. These chains vary significantly between antibodies. The variable regions make antibodies antigen-specific. Constant regions, on the other hand, are relatively the same between antibodies. All antibody molecules, whether made in response to a chickenpox virus or to a Salmonella bacterium, have some regions that are similar.