State the number of polypeptide chains in $H_{2}L_{2}$ (an antibody molecule).

Acquired immunity is mediated by which of the following?

After maturation,in primary lymphoid organs,the lymphocytes migrate for interaction with antigens to secondary lymphoid organs like:
$A.$ thymus $B.$ bone marrow $C.$ spleen $D.$ lymph nodes $E.$ Peyer's patches
Choose the correct answer from the options given below:

Which of the following organs provides the sites for interaction of lymphocytes with the antigen?

Given below are two statements:
Statement $I$: Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced.
Statement $II$: Both bone marrow and thymus provide micro-environments for the development and maturation of $T$-lymphocytes.
In the light of the above statements,choose the most appropriate answer from the options given below:

What is an antigen?