# Explain how the immune system and the lymphatic system are integrated in the body's response to a bacterial infection.

> IB DP Biology Standard Level (2023 syllabus) — C3.1 Integration of body systems · Explain · 4 marks

## Mark scheme (4 marks)

1. Lymphatic vessels carry interstitial fluid (as lymph) through lymph nodes, where immune cells are concentrated / where pathogens can be intercepted.
2. Macrophages / dendritic cells in lymph nodes engulf bacteria by phagocytosis and present antigens to lymphocytes / T cells.
3. B lymphocytes in lymph nodes are activated and differentiate into plasma cells that secrete antibodies specific to the bacterial antigen.
4. Lymphocytes proliferate within lymph nodes and are distributed via the lymphatic system back into the bloodstream / circulatory system, providing a systemic immune response.

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