Immature lymphocytes differentiate in which of the following organs?

Memory cells are responsible for:

Match the antibodies in List-$I$ with their respective functions in List-$II$.

List-$I$	List-$II$
$(a)$ $IgG$	$(i)$ Protection of body surfaces
$(b)$ $IgA$	$(ii)$ Regulation of allergic reactions
$(c)$ $IgM$	$(iii)$ Activates $B$-cells
$(d)$ $IgD$	$(iv)$ Protection of body fluids
$(e)$ $IgE$	$(v)$ Protection of blood circulation

Non-specific host defence that exists prior to the exposure to an antigen is called

What percentage of lymphoid tissue in the human body is constituted by $MALT$ (in $\%$)?

When a pathogen (antigen) enters the body for the second time,which type of antibody is produced in the secondary immune response?