IMMUNOLOGY

Detailed explanation-1: -Normally, thyroid gland is devoid of lymphoid tissue while this is reversed during the course of autoimmune diseases such as Hashimoto’s thyroiditis.

Detailed explanation-2: -Primary lymphoid organs: These organs include the bone marrow and the thymus. They create special immune system cells called lymphocytes. Secondary lymphoid organs: These organs include the lymph nodes, the spleen, the tonsils and certain tissue in various mucous membrane layers in the body (for instance in the bowel).

Detailed explanation-3: –Earlier the appendix used to be considered as a vestigial organ, but nowadays this view has changed. Recent research suggests that the appendix may serve a crucial purpose as a reservoir for beneficial gut bacteria. Therefore, option (D) is correct.

Detailed explanation-4: -This system includes the bone marrow, spleen, thymus, lymph nodes, and lymphatic vessels (a network of thin tubes that carry lymph and white blood cells). Lymphatic vessels branch, like blood vessels, into all the tissues of the body.