Q.15 Lymphocytes interact with foreign antigens in

• (A) Bone marrow
• (B) Peripheral blood
• (C) Thymus
• (D) Lymph nodes

  Lymphocytes (B and T cells) patrol the body to detect foreign antigens via specific receptors. While they circulate continuously, productive immune activation occurs in organized lymphoid structures. This article solves the MCQ, confirming (D) Lymph nodes as the primary interaction site.

  Lymphocyte Trafficking and Antigen Encounter

  Naive lymphocytes exit bone marrow (B cells) or thymus (T cells) and recirculate through blood and lymph. Antigens draining from tissues via lymphatics are captured by antigen-presenting cells (APCs) like dendritic cells in lymph nodes. Here, high endothelial venules concentrate naive lymphocytes for scanning APCs bearing peptide-MHC complexes.

  Correct Answer: (D) Lymph nodes
  Lymph nodes serve as secondary lymphoid organs where naive lymphocytes first productively engage foreign antigens. T cells recognize MHC-peptide on dendritic cells; B cells bind native antigens via BCR in follicles.

  Explanation of All Options

  Each location’s role in lymphocyte activation:

  • (A) Bone marrow
    Incorrect. Primary site for B cell development and maturation. Mature B cells emigrate; no significant foreign antigen encounter here.

  • (B) Peripheral blood
    Incorrect. Lymphocytes transit blood during recirculation but don’t productively interact with antigens. APCs process antigens in tissues/lymph nodes first.

  • (C) Thymus
    Incorrect. Primary site for T cell maturation and negative selection. No foreign antigen exposure—focuses on self-tolerance.

  • (D) Lymph nodes
    Correct. Organized structure with T cell zones (paracortex) and B cell follicles where APCs present antigens to circulating lymphocytes.

  Option	Lymphoid Type	Antigen Interaction?	Primary Function
  (A) Bone marrow	Primary	No	B cell development
  (B) Peripheral blood	Circulation	Transient	Lymphocyte trafficking
  (C) Thymus	Primary	No	T cell maturation
  (D) Lymph nodes	Secondary	Yes	Antigen-driven activation

  Immunology Applications

  Lymph nodes amplify immune responses via germinal centers (B cell affinity maturation) and T cell priming. Vaccine adjuvants target lymph node delivery; cancer immunotherapy blocks checkpoints here. For biotech students, understanding trafficking aids vaccine design and flow cytometry analysis of activation markers (CD69, CD25).