The complement system's classical pathway requires:

Correct Answer: A

Rationale: The classical pathway needs antibodies; pathogens trigger alternative, cytokines signal, histamines inflame.

Correct Answer: B

Rationale: B-lymphocyte education occurs in bone marrow; the thymic medulla is for T-cell maturation.

Correct Answer: B

Rationale: Lymphatic capillaries remove fat and bacteria due to gaps in endothelium and interrupted basal lamina, not tight junctions or non-fenestrated cells.

Correct Answer: A

Rationale: Carcinoma causes lymph node enlargement via metastatic spread from a primary site.

Correct Answer: D

Rationale: Palatine tonsils have non-keratinized epithelium , glands opening into crypts , and nodules ; they are non-capsulated but prone to inflammation (D is incorrect).