Which of the following is not a physiological barrier?

Match the columns:

Column-$I$	Column-$II$
$(a)$ Main lymphoid organ	$(1)$ Thymus
$(b)$ $MALT$	$(2)$ Spleen
$(c)$ Lobe-like organ arranged near heart	$(3)$ Bone marrow
$(d)$ Large bean-shaped organ	$(4)$ It constitutes $50\%$ of lymphoid tissue

Match List-$I$ with List-$II$:

List-$I$	List-$II$
$(a)$ Antibody	$(i)$ Cell-mediated immunity
$(b)$ $B$-lymphocytes	$(ii)$ Humoral immune response
$(c)$ $T$-lymphocytes	$(iii)$ $H_2L_2$
$(d)$ $T$-cells help $B$ cells to produce them	$(iv)$ Army of proteins

$A$ molecule that elicits an immune response is called

$A$: Live attenuated vaccine is better in terms of immunity provided to the recipient.
$R$: As secondary lymphoid organs,example $Peyer's$ patches are stimulated to protect the individual.

How many polypeptide chains are present in each antibody molecule?