$.........$ is responsible for graft rejection.

In case of snakebites,the injection given to patients contains preformed antibodies against the snake venom. This type of immunisation is called . . . . . . .

Which of the following is not present in the structure of an antibody?

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

Which antibody has the highest number of antigen-binding sites?

Match the columns:

Column-$I$	Column-$II$
$(a)$ Physical barrier	$(1)$ Skin
$(b)$ Physiological barrier	$(2)$ Interferon protein
$(c)$ Cellular barrier	$(3)$ Leucocytes
$(d)$ Cytokine barrier	$(4)$ Saliva in the mouth