Innate immunity
Introduced| English | Meaning |
|---|---|
| complement/ˈkɒmplɪmənt/ | A cascade of blood proteins that tags microbes, attracts phagocytes and punches holes in membranes. |
| neutrophils | The most common white blood cells, first to arrive in acute inflammation. |
| macrophages | Large phagocytes in tissues that engulf microbes and debris and release signals that start inflammation. |
| natural killer cells | Innate lymphocytes that kill virus-infected and tumour cells, especially those that have lost their MHC class I. |
| pattern recognition receptors | Innate immune receptors, such as the Toll-like receptors, that recognise molecular patterns shared by many microbes. |
| dendritic cells | Antigen-presenting cells that capture antigen in tissues and carry it to lymph nodes to activate naive T cells. |
Red, hot and swollen by lunchtime
- A splinter goes in at 9 a.m. By lunchtime the finger is red, warm and swollen, and white cells are already fighting, days before any antibody exists.
- That first defence is innate immunity: you are born with it, it acts in minutes to hours, and it never learns.
Barriers and complement
- Skin, stomach acid and mucus stop most microbes. Behind them is complement 补体, a set of blood proteins that coat microbes so phagocytes eat them, and punch holes in some membranes.
- Complement can start without any antibody (the alternative and lectin pathways) or be triggered by antibody (the classical pathway).
How can complement be activated before any antibody has been made?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Only the classical pathway is triggered by antibody; the other two start on microbial surfaces.
Neutrophils: first and short-lived
- Neutrophils 中性粒细胞 are 50–70% of white blood cells (normal count about 2–7 × 10⁹/L). They squeeze out of the blood within the first hour, dominate the first 6–24 hours, and die in the pus.
- They eat bacteria and fungi and kill them with enzymes and oxygen radicals.
Macrophages and NK cells
- Monocytes leave the blood and become macrophages 巨噬细胞, which take over at 24–48 hours, clear debris and show antigen to T cells.
- Natural killer cells NK细胞 are innate lymphocytes. They kill virus-infected and stressed cells, especially cells that have lost their MHC class I, without needing antibody.
Put the response to a cut in the skin in order of time.
Difficulty 3/5 · 200 coins + a small bonus on first completion today
Innate cells hold the line for days while the adaptive response is built.
NK cells need antibody before they can kill a virus-infected cell.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
NK cells kill stressed cells and cells that have lost MHC class I directly; antibody can help but is not required.
Fixed receptors for common patterns
- Innate cells use pattern recognition receptors 模式识别受体: receptors coded directly in the genome that bind patterns shared by many microbes, such as bacterial lipopolysaccharide binding TLR4.
- No genes are rearranged, so the response is fast and the same every time; there is no memory.
Why do innate responses have no memory?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Memory comes from expanding clones with rearranged receptors; innate receptors are the same every time.
Dendritic cells carry the news
- Dendritic cells 树突状细胞 sample antigen in the tissues, travel to a lymph node and show it to T cells, together with the second signal that switches T cells on.
- So innate immunity cannot learn, but it decides whether the adaptive response starts at all.
Match each innate component to its job.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Each cell has its own timing and job.
Worked example: the marrow's output
- The marrow makes about 10¹¹ neutrophils a day in health. A day has 1440 minutes, so that is $10^{11} \div 1440 \approx 7 \times 10^{7}$, about 70 million a minute.
- In severe infection the marrow raises that several-fold, which is why a high neutrophil count is a sign of bacterial infection.
The marrow makes 10¹¹ neutrophils a day. How many million is that per minute?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
10¹¹ ÷ 1440 ≈ 6.94 × 10⁷, about 69 million a minute.
Worked example: fever without neutrophils
- (Fictional) A patient on chemotherapy has a fever and a neutrophil count of 0.3 × 10⁹/L, about a seventh of the lower limit of 2.
- The first responders are gone and the adaptive response is days too slow, so sepsis can develop within hours. Febrile neutropenia is treated as an emergency.
A neutrophil count of 0.3 × 10⁹/L is what percentage of the lower normal limit of 2 × 10⁹/L?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
0.3 ÷ 2 = 0.15 = 15%, severe neutropenia.
Three common slips
- "NK cells are adaptive." They are innate lymphocytes with no rearranged receptor.
- "Complement needs antibody." The alternative and lectin pathways start without it.
- "Innate immunity just waits for the real response." Its dendritic cells decide whether that response happens.
Recap
- Barriers, then complement, then neutrophils (first 6–24 hours), then macrophages (24–48 hours).
- NK cells kill infected cells without antibody; pattern recognition receptors are fixed, with no memory.
- Dendritic cells carry antigen and the "go" signal to T cells.