This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Which of the following immunoglobulins is rich in serum and body secretions? 
Select and explain. 
A. Ig G, B. IgA, C. Ig M, D. IgD
B: IgA
Answer: B: IgA 
Explanation: IgA is the second most abundant immunoglobulin constituting 10-13% of the total. It is the predominant immunoglobulin in seromucous secretions such as saliva, tears, colostrum, breast milk and tracheobronchial and genitourinary secretions. In human sera more than 80% of IgA occurs as monomer containing 2H and L chains but in seromucous secretions it occurs as a dimer cing 4H and 4L chains and is known as secretory IgA.