Which immunoglobulin is primarily measured in the radioallergosorbent (RAST) test?

The radioallergosorbent test (RAST) is specifically designed to detect the presence of Immunoglobulin E (IgE) in the serum, which plays a crucial role in allergic reactions and responses to allergens. IgE binds to allergens and triggers the release of histamines and other chemicals from mast cells, leading to allergy symptoms. This test is particularly useful for diagnosing various allergic conditions by determining the level of IgE antibodies against specific allergens.

The other immunoglobulin classes, such as IgA, IgM, and IgD, serve different functions in the immune system. IgA is primarily involved in mucosal immunity, found in secretions like saliva and tears; IgM is typically present during the initial immune response and is effective in agglutinating pathogens; while IgD is primarily found on the surface of B cells and is involved in the early stages of the immune response. However, none of these immunoglobulins are the focus of the RAST test. Therefore, the determination of IgE is critical for assessing allergic sensitivities, making it the correct answer in the context of the RAST test.