Immunology

Naturally acquired passive immunity

Step 1: Passive immunity means antibodies are received from an outside source — the baby does not produce them itself. Step 2: Since the antibodies come from the mother's colostrum (first milk), this happens naturally, not through artificial means like injection. Step 3: So the immunity is both 'passive' (antibodies from outside) and 'naturally acquired'. Step 4: Active immunity would require the baby's own immune system to produce antibodies, which is not the case here. Step 5: Artificial immunity involves vaccines or injections given by doctors, not natural feeding.

Antibody levels rise steeply and much higher than in the primary response

Step 1: The secondary immune response occurs when a person is exposed to the SAME antigen for the second time. Step 2: Because immunological memory has been established, the immune system recognizes the antigen quickly. Step 3: Within 2-3 days, antibody concentration rises steeply to much higher levels than in the primary response. Step 4: This rapid and stronger response is why it is called the 'booster response'. Step 5: Options A and C are incorrect descriptions of primary response features, and Option D is wrong because memory cells are the KEY feature of secondary immunity.

Thymus gland

Step 1: Both B and T lymphocytes are produced in the bone marrow. Step 2: B lymphocytes mature within the bone marrow itself. Step 3: T lymphocytes, however, leave the bone marrow and travel to the thymus gland to mature. Step 4: In the thymus, T cells (called thymocytes) become immunocompetent (capable of mounting an immune response). Step 5: Spleen and lymph nodes are secondary lymphoid organs where mature lymphocytes accumulate — they are not sites of T cell maturation.

Thymus

Step 1: Lymphoid organs are classified as primary (central) and secondary (peripheral). Step 2: Primary lymphoid organs are where lymphocytes are produced and matured — they include the bone marrow and the thymus gland. Step 3: Thymus is a primary lymphoid organ where T cells mature. Step 4: Spleen, lymph nodes, and tonsils are all SECONDARY (peripheral) lymphoid organs — they trap antigens and expose them to mature lymphocytes. Step 5: Therefore, among the given options, only Thymus is a primary lymphoid organ.