Which of the following is the first immunoglobulin to appear following infection?

IgM is the first immunoglobulin to appear after exposure to an antigen. They are relatively sho lived hence their demonstration in the serum indicates recent infection. Normal level of IgM is 1.2mg/ml. It is also the earliest to be synthesized by fetus beginning by about 20 weeks of age. As it cannot cross the placental barrier its presence in fetus or newborn indicates intrauterine infection. Its detection is therefore useful for diagnosis of congenital infections such as syphilis, rubella, HIV and toxoplasma. IgG is the major immunoglobulin in human serum. It is the only class of immunoglobulins which can cross the placenta. It binds to organisms and enhances their phagocytosis. It paicipates in most immunologic reactions such as complement fixation, precipitation, and neutralisation of toxins and viruses. Ref: Textbook of Microbiology By Surinder Kumar page 138.