Immunoglobulins (Igs) are proteins contributing to immunity mechanisms. Immunity is specific to the lymphoid system, which consists of organs (spleen, thymus, bone marrow) and cells (lymphocytes). Immunoglobulins are secreted by B lymphocytes in the blood, and in this way are exported for the specific functions of humoral immunity.
Immunoglobulin M is the most primitive and least specialised immunoglobulin. It is also the only Ig synthesised normally by newborns. The majority of serum IgMs are pentamers. They are the first Igs that appear during immune the response.