Q.Draw a labelled diagram of immunoglobulin. [figure: a labelled diagram of an immunoglobulin (antibody) molecule — the printed question asks the student to draw and label it themselves, no figure is printed in the question paper]

An immunoglobulin is drawn as a Y-shaped structure of two heavy and two light polypeptide chains linked by disulfide bonds, with antigen-binding sites at the tips of the arms.

To label the diagram correctly: draw two identical longer 'heavy chains' running the full length of the Y (forming the stem and continuing into both arms) and two identical shorter 'light chains' paired alongside the upper part of each arm; connect the chains with several interchain disulfide bonds (S-S bridges), with additional intrachain disulfide bonds folding each chain into compact domains. Label the tip of each arm — where one heavy chain and one light chain meet — as the 'variable region' (VH and VL), which together form the antigen-binding site; label the remaining, unchanging part of each chain as the 'constant region' (CH and CL). Mark t …