Questions: Lymphatic System Anatomy and Immune Surveillance

Question 1

A patient develops severe lymphedema in their arm following a mastectomy that included removal of axillary lymph nodes. What is the direct cause of the swelling?

• A. The removed nodes are no longer producing proteins that maintain osmotic balance in the arm
• B. Disrupted lymphatic drainage prevents interstitial fluid from being returned to circulation, so it accumulates in the tissue
• C. The immune system can no longer fight infection in the arm, causing inflammatory fluid buildup
• D. Loss of lymph nodes reduces blood flow to the arm, causing fluid to leak from blood vessels

Answer: (B)

Lymphatic capillaries collect the ~3 L/day of plasma that seeps from blood capillaries into interstitial space. When lymphatic drainage is disrupted — whether by surgery, tumor blockage, or infection — this fluid cannot return to circulation and accumulates as edema. The misconception in option A confuses lymph nodes (immune surveillance organs) with glands that produce osmotic proteins; they do neither.

Question 2

After a pathogen enters the foot, where does clonal selection and proliferation of antigen-specific lymphocytes most likely occur?

• A. In the bone marrow, where B lymphocytes are produced and can immediately respond to antigen
• B. In the thymus, which activates mature T cells when infection signals reach it
• C. In the inguinal lymph nodes, which drain the lower limb and house T and B cells awaiting antigen encounter
• D. In circulating blood, where lymphocytes patrol for antigen and activate upon contact

Answer: (C)

The inguinal lymph nodes are secondary lymphoid organs strategically positioned to intercept antigen draining from the lower limbs. Resident dendritic cells sample lymph as it percolates through, and antigen-specific T and B cells in the node undergo clonal selection and expansion. The thymus (option B) and bone marrow (option A) are primary lymphoid organs — sites of lymphocyte development and education, not antigen encounter. Lymphocyte activation primarily occurs in lymphoid tissue, not free-floating in blood (option D).

Question 3

Lymph flows without a dedicated pump equivalent to the heart.

• T. True
• F. False

Answer: True

Lymph flow depends entirely on skeletal muscle contractions during movement, pressure changes during breathing, and smooth muscle contractions in larger lymphatic walls — not a dedicated pump. This is why immobility (bed rest, paralysis) leads to lymphedema: without the mechanical forces that drive lymph through the vessels, interstitial fluid accumulates. This also explains why exercise is therapeutic for lymphedema patients.

Question 4

The thymus is where both T and B lymphocytes undergo positive and negative selection to eliminate cells that would attack the body's own tissues.

• T. True
• F. False

Answer: False

Only T lymphocytes mature in the thymus, where positive selection (keeping cells that can recognize self-MHC) and negative selection (eliminating self-reactive cells) occur. B lymphocytes mature and undergo central tolerance selection in the bone marrow. The thymus and bone marrow are the two primary lymphoid organs, each dedicated to educating a different lymphocyte lineage.

Question 5

Why do lymph nodes swell during an active infection, and what does this swelling indicate about the immune response?

Model answer: Lymph nodes swell because lymphocytes within them — particularly T cells in the deep cortex and B cells in the follicles — are proliferating and differentiating in response to antigen delivered by draining lymph. The enlargement reflects active clonal expansion: many new immune cells are being produced to match the scale of the threat.

Swollen lymph nodes are clinically valuable because their anatomical location identifies the source of infection — cervical nodes implicate head and neck structures, axillary nodes implicate arm or breast, inguinal nodes implicate lower limb. The swelling is not a sign of a failing immune system but of one actively mounting an adaptive response. Nodes that remain enlarged long after infection may indicate persistent antigen (chronic infection, malignancy) or a primary lymphoid malignancy.