You met the body's defenses in The Body's Defenses. They work in two layers, which differ in speed and in aim. This topic builds on the teamwork of body systems: immune cells travel in the blood and lymph.
Key idea
Innate defenses act fast against anything foreign: skin and mucus, inflammation, and white blood cells such as macrophages that engulf germs. Adaptive defenses take days the first time but target one particular invader: B cells make antibodies, and T cells destroy infected cells and direct the response. Adaptive defenses leave memory cells, so a second encounter brings a faster, stronger response.
Why it works
Why two layers? Innate defenses are ready at once but cannot tell one germ from another. Adaptive defenses take time because the body must find, among millions of B and T cells with different receptors, the few that fit this invader, and then multiply them. That search is slow the first time. Afterward, some of those cells stay as memory cells, so the search is already done.
Worked example: Innate or adaptive?
A macrophage engulfs bacteria that entered a scraped knee, within hours. Which defense is this?
- It acts quickly.
- It attacks bacteria in general, not one particular kind.
Answer: Innate.
Worked example: Reading the response
In the chart, how many days after each exposure does the antibody level peak?
- First exposure (day 0): the peak is at day 14, so 14 days.
- Second exposure (day 35): the peak is at day 49, also 14 days later, but about eight times higher; and antibodies are already rising by day 42.
Answer: Both about 14 days, but the second response rises sooner and much higher.
Vaccines work by this memory. In the CDC's words, "vaccines help develop immunity by imitating an infection": they show the immune system a harmless form or part of a germ, so memory cells form without the illness. Antibiotics are a different tool: they act on bacteria and do not work on viruses.
Common mistake
Two misunderstandings are common. Antibodies are not cells; they are Y-shaped proteins made by B cells, which bind to one particular invader. And the immune system does not forget every invader: memory cells make a second response faster and stronger.