Read the following passage carefully and answer the given questions.

Immune System in the Body

The human immune system consists of lymphoid organs, tissues, cells and soluble molecules like antibodies. The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes. After maturation the lymphocytes migrate to secondary lymphoid organs like spleen, lymph nodes, tonsils, Peyer's patches of small intestine and appendix. The secondary lymphoid organs provide the sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells. The bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced. Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as $H_2L_2$. Different types of antibodies are produced in our body. IgA, IgM, IgE, IgG are some of them. Techniques like radiography (use of X-rays), CT (computed tomography) and MRI (magnetic resonance imaging) are very useful to detect cancers of the internal organs.

Human immune system recognizes the viral proteins as:

Antigen

The correct answer is Option (3) → Antigen 

Viral proteins are foreign molecules to the human body. The immune system identifies them as antigens, which then trigger the production of antibodies to fight the infection.

• Antigen = any foreign molecule (like viral protein) that stimulates an immune response.
• Antibody = the defense protein produced by our body against the antigen.
• Antibiotic = drug used to kill/inhibit bacteria (not viruses).
• Soluble immune molecule = general term, not specific here.