Short answers, pulled from the story.
An antibody, also called an immunoglobulin, is a large Y-shaped protein produced by B cells that identifies and neutralizes antigens such as those found on bacteria and viruses. Antibodies work by tagging pathogens for destruction by immune cells, neutralizing pathogens directly by blocking their ability to infect cells, and activating the complement system to destroy bacteria.
Humans can generate an estimated 10 billion different antibodies, each capable of binding a distinct epitope on an antigen. This diversity is achieved through a gene-assembly process called V(D)J recombination, which randomly combines variable, diversity, and joining gene segments in each developing B cell.
The five classes of human antibodies are IgA, IgD, IgE, IgG, and IgM. IgG provides the majority of antibody-based immunity and is the only class that crosses the placenta; IgA protects mucosal surfaces; IgE is involved in allergic responses; IgM acts early in infections before IgG levels rise; and IgD functions mainly as an antigen receptor on naive B cells.
Gerald Edelman and Rodney Porter were jointly awarded the 1972 Nobel Prize in Physiology or Medicine for deducing the structure and complete amino acid sequence of IgG. Edelman discovered that antibodies are composed of disulfide bond-linked heavy and light chains, while Porter characterized the Fab and Fc regions.
The term Antikörper, the German word for antibody, first appeared in Paul Ehrlich's article published in October 1891. The study of antibodies began a year earlier, in 1890, when Emil von Behring and Kitasato Shibasaburo described antibody activity against diphtheria and tetanus toxins.
Monoclonal antibody therapies are used to treat rheumatoid arthritis, multiple sclerosis, psoriasis, non-Hodgkin's lymphoma, colorectal cancer, head and neck cancer, and breast cancer. They are also used to treat immune deficiencies such as X-linked agammaglobulinemia and hypogammaglobulinemia through a form of passive immunity.