What are $IgA$ and $IgM$?

Identify the cells that assist $B$-lymphocytes in the production of antibodies.

Where are $B$-cells and $T$-cells formed? How do they differ from each other?

The immune system which works against self is

Which of the following are secondary lymphoid organs?
$(a)$ Bone marrow
$(b)$ Tonsils
$(c)$ Spleen
$(d)$ Thymus
Choose the correct answer from the options given below:

The antigen-binding site in an antibody molecule is formed by: