Question

The class of antibodies which appear first after the infection, is? 
A) IgG                                         
B) IgM
C) IgA                                         
D) IgD

Answer 1

Hint: It is the first class of antibody made by a developing B cell. It accounts for 5-10% of total serum Ig. It is also found on the surface of mature B cells together with IgD where it serves as an antigen-specific B cell receptor. It has a molecular mass of 180 kDa. It is secreted by plasma cells as a pentameric molecule.

Complete step-by-step answer:

 In mammals, there are five classes or isotypes of antibodies or immunoglobulins, IgA, IgD, IgG, IgE, and IgM, each with its class of heavy chain. The fully assembled antibody derives its name from the heavy chain.

IgD: No effector function has been identified for IgD. IgA is the major immunoglobulin found in the colostrums of milk in nursing mothers, and It may provide the neonate with a major source of protection against pathogens during the first few weeks after birth.

IgG is the only class of antibodies that can pass through the placenta. Transcytosis of IgG antibodies from the maternal circulation across the placenta delivers maternal antibodies to the fetus.

 IgM is the first antibody class produced in the primary response to an antigen, and it is also the first antibody to be synthesized by the neonate. IgM is more efficient than IgG in activating the complement system. Complement activation requires two Fc regions nearby, and the pentameric structure of a single molecule of IgM fulfills this requirement.

Therefore the correct answer is Option B.

Note: The valency of an antibody refers to the number of antigenic determinants that an antibody molecule can bind. The valency of antibodies is at least two (bivalent) and in some instances more (multivalent). The monomeric form of all antibody classes is homobivalent. Dimeric and pentameric forms of antibodies are multivalent.