Question

1- why IgA does not activate complement ?

2- why IgG2 does not cross placenta ?

Answer 1

immunoglobulin A (IgA)

it is found in the lining of respiratory tract and digestive system as well as in saliva ,tears ,and breast milk.

The complementary system also known as complement cascade ,is a part of the immune system that enhance the ability of antibodies and phagocytic cells to clear microbs and damaged cells from an organism,promote inflammation,and attacks the pathogen's cell membrane

Activation of the complement (c)system by human IgA1 and IgA2 and secreory IgA were shown to activate c,as determined by deposition of c3 on glutaraldehyde-activated microwells coated with IgA.The activation of the c system occured in the presence of MgEGTA and not in D -DEFICIENT SERUM.

Reduction and alkylation destroyed the ability of IgA to activate C .the four chain molecules F(ab)2 and F(abc)2 were shown to activate the ALTERNATIVE PATHWAY (AP ) But no activation was observed with the two -chain fragments Fab and Fc.The Fc fragment of IgA also did not activate the CP, as does the Fc fragment of IgG.this indicate that activation of the AP of C by IgA is dependent on the presence of the F(ab)2 fragment .Thus human IgA DOES NOT ACTIVATE C   by the AP.this activation requires an intact f(ab)2 fragment

question 2  

The IgG2 subclass cannot cross the placenta and neutralize antigens ;therefore ,infection related to these antigen may persist and can leads to serious conditions like miscarriage and stillbirth.The  unique short -hinged structure of IgG2 ,WHICH ENABLES ITS K-,BUT NOT ISOTYPE TO FORM AT LEAST THREE DIFFERENT STRUCTURAL isoforms ,might be a contributing factor to these difference.