What is the main lymphoid organ where all blood cells including lymphocytes are produced?

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 of the following are included as cellular barriers in innate immunity?

What are $IgA$ and $IgE$?

Passive acquired immunity is different from active acquired immunity because:

Which of the following cells has the longest lifespan?