In hyperviscosity syndrome, which immunoglobulin class is most likely involved in monoclonal disease?

Hyperviscosity syndrome is often associated with the presence of an increased concentration of immunoglobulins in the blood, typically related to monoclonal gammopathies. Among the classes of immunoglobulins, IgM is particularly notable in this context because it is a pentamer consisting of five monomer units, which leads to a larger molecular size compared to other immunoglobulin classes. When produced in excess due to conditions such as Waldenström's macroglobulinemia or certain lymphoproliferative disorders, IgM can significantly increase the viscosity of the blood, causing various symptoms and complications.

IgA and IgG, while they can also be involved in monoclonal diseases, do not typically cause hyperviscosity to the same extent as IgM due to their smaller molecular size. IgE is more commonly associated with allergic reactions and is not usually relevant in the context of hyperviscosity syndrome. Therefore, the association of IgM with hyperviscosity is a critical aspect of the condition, making it the correct answer in this scenario.