A) \[IgM\]
B) \[IgG\]
C) \[IgA\]
D) none of these.
Correct Answer: A
Solution :
:\[IgM\] is the largest of the immunoglobulins and the first to reach the site of infection being present in the circulating fluid. When a B- lymphocyte is first stimulated by antigen\[IgM\] is the first antibody type to be produced during an immune response.\[IgM\]fixes complement and does not cross placenta. Later on\[IgG\] replaces\[IgM\].\[IgG\]is the main antibody in the secondary response. It crosses the placenta. \[IgA\]is the predominant antibody class in seromucous secretions such as saliva, tears, colostrums and respiratory, gastrointestinal and urinogenital secretions.\[IgA\]provides the first line of defence against invading pathogens, inhibiting their adherence to the mucosal cells and to preventing their invasions of the body.
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