For instance, one B lymphocyte will make an antibody to block a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia. When a B lymphocyte encounters its triggering antigen, it gives rise to many large cells known as plasma cells. Every plasma cell is essentially a factory for producing a unique antibody. Each of the plasma cells descended from a given B lymphocyte manufactures millions of identical antibody molecules and pours them into the bloodstream. Antibodies belong to a family of large molecules known as immunoglobulins.
Different types play different roles in the immune defense strategy.
Immunoglobulin G, or IgG, works efficiently to coat microbes, speeding their uptake by other cells in the immune system. Immunoglobulin M is very effective in killing bacteria. Immunoglobulin E, whose natural job probably is to protect against parasite infections, is responsible for the symptoms of allergy. When the immune system is missing one or more of its components, the result is an immunodeficiency disorder. Temporary immune deficiencies can develop in the wake of common virus infections, including influenza, infectious mononucleosis and measles.