Immune System

Step 1: Understanding the Concept: The human immune system relies on lymphoid organs, which are broadly categorized into primary and secondary organs based on their specific function in lymphocyte development.
Step 2: Key Formula or Approach: Differentiate between primary organs (sites of maturation) and secondary organs (sites of interaction), and scan the options to exclude any primary organs.
Step 3: Detailed Explanation: Primary lymphoid organs are the foundational sites where immature lymphocytes originate, mature, and become antigen-sensitive. The only two primary lymphoid organs in humans are the Bone Marrow and the Thymus.
Secondary lymphoid organs are the functional sites where these mature lymphocytes migrate to interact with antigens, subsequently proliferating to become effector cells. Classical examples include the Spleen, Lymph nodes, Tonsils, Peyer's patches of the small intestine, and the Appendix.
Looking at the given options:
Option (1) contains Thymus (Primary).
Option (3) contains Bone marrow (Primary).
Option (4) contains Thymus (Primary).
Option (2) consists solely of Lymph nodes, Appendix, and Tonsils, all of which are correctly classified as secondary lymphoid organs.
Step 4: Final Answer: The correct option is (2).