CD40LG and infection: There are five primary classes of Igs: (i) IgA is the second most common in human serum and is most prevalent in human secretions, such as saliva, tears, etc.; (ii) IgD is found in very low concentrations in human serum and the least understood; (iii) IgE is least abundant in human serum; (iv) IgG is the most common in human serum; (v) IgM is the third most common in human serum, but is expressed at the early stages of infection, i.e., on the surface of immature and mature B cells.