English: A basic diagram showing the three layers of defense in the human immune system in response to a bacterial infection.

1. Skin, and other physical barriers 2. Laceration, or break in the barrier 3. Harmful Bacteria 4. Macrophage 5. Macrophage envelops Bacteria, destroys it, and identifies the Antigens present 6. APC, or Antigen Presenting Cell, carries the Antigens of the bacteria 7. Helper T Cell transfers Antigens from APC to B Cell 8. B Cell is activated by Helper T Cell 9. Some B Cells become Memory B Cells, ready to provide antigens for future infection 10. Other B Cells become Plasma Cells, that secrete antibodies 11. Antibody 12. Antibodies match with Antigens present on the Bacteria, disabling them, and stopping the infection

I. Non-Specific (Barriers) II. Non-Specific (General Responses)

III. Specific Response