Immune System

Step 1: Understanding the Terms

Each item in List-1 represents a specific type of biological structure, while List-2 provides corresponding components.
Step 2: Matching the Terms Correctly

- Primary lymphoid organ refers to organs responsible for the production and maturation of immune cells. The Bursa of Fabricius (II) is a primary lymphoid organ in birds.
- Secondary lymphoid organ includes sites where immune responses occur. Tonsils (IV) are secondary lymphoid organs.
- Mononuclear phagocytes include cells that engulf pathogens. Kupffer cells (I) are liver macrophages and belong to this category.
- Auxiliary cells support immune responses, and Mast cells (V) play a role in allergic reactions.

Step 3: Conclusion

Thus, the correct matching is:
- A → II
- B → IV
- C → I
- D → V

Step 1: Classification of Lymphoid Organs:
Lymphoid organs are classified into two types:

• Primary Lymphoid Organs: Sites where immature lymphocytes differentiate and mature into antigen-sensitive lymphocytes. Examples: Bone Marrow and Thymus.
• Secondary Lymphoid Organs: Sites where lymphocytes interact with antigens and proliferate. Examples: Spleen, Lymph nodes, Tonsils, Peyer's patches, MALT.

Step 2: Evaluate Options:
Among the given options, the Thymus gland is a primary lymphoid organ. The others are secondary.

• Monocytes that migrate from blood into connective tissue differentiate into macrophage-type cells often referred to as histiocytes.
• Kupffer cells (note spelling) are liver macrophages (specialised), microglia are CNS macrophages — both are tissue macrophages but derived from monocytes and specific to organs.
• Memory cells are lymphocytes (not monocyte derivatives).
• Hence the correct answer is (1) Histiocytes.

B-lymphocytes (B-cells) differentiate into plasma cells which secrete antibodies (immunoglobulins) specific to antigens. T-lymphocytes provide cell-mediated immunity but do not produce antibodies. TH cells (helper T) stimulate B-cells. NK cells kill virus-infected or tumor cells.