How many peptide chains are present in an antibody molecule?

Why does immunity in old people decrease?

Which of the following is not a type of $T$-lymphocyte?

After birth,an infant receives antibodies $IgA$ through colostrum from the mother. This is an example of . . . . . . immunity.

Which one of the following options gives the correct categorization of lymphocytes according to formation,maturation,and type of immunity?

Match the type of immunity listed in Column-$I$ with the examples listed in Column-$II$. Choose the answer that gives the correct combination of alphabets of the two columns:

Column-$I$ (Types of immunity)	Column-$II$ (Example)
$A$. Natural active	$p$. Immunity developed by heredity
$B$. Artificial passive	$q$. From mother to foetus through placenta
$C$. Artificial active	$r$. Injection of antiserum to travellers
$D$. Natural passive	$s$. Fighting infections naturally
	$t$. Induced by vaccination