Innate vs Adaptive Immunity
The fast generic wall vs the slow specific memory
Two arms of immunity
Fast-generic vs slow-specific. Innate immunity acts within minutes to hours, is nonspecific, uses germline-encoded receptors, and has no memory. Adaptive immunity is slow on first exposure (days), highly specific, and generates lasting memory. The two are not independent: innate immunity activates and instructs the adaptive response.
Adaptive arms. Adaptive immunity has a cell-mediated arm (T cells) and a humoral arm (B cells and antibodies). Its enormous receptor diversity comes from V(D)J recombination of antigen-receptor genes, catalyzed by RAG1 and RAG2. Memory lets a second exposure produce a faster, stronger recall response.
| Feature | Innate | Adaptive |
|---|---|---|
| Speed | Minutes to hours | Days on first exposure |
| Specificity | Conserved microbial patterns (PAMPs) | Individual antigens |
| Receptors | Germline-encoded PRRs (e.g., TLRs) | TCR and BCR from V(D)J recombination |
| Memory | None (response identical each time) | Yes; recall is faster and stronger |
| Cells | Neutrophils, macrophages, dendritic cells, NK cells, mast cells, basophils, eosinophils | T cells, B cells, plasma cells |
| Humoral component | Complement | Antibodies |
Innate recognition and effectors
The first responders. Innate defenses include epithelial barriers, complement, and the innate cells: neutrophils, macrophages, dendritic cells, NK cells, mast cells, basophils, and eosinophils. They recognize conserved pathogen-associated molecular patterns (PAMPs) through pattern-recognition receptors (PRRs) — the PAMP is the microbial motif, the PRR is the host receptor that reads it.
Toll-like receptors (TLRs). The best-known PRRs. Surface TLRs sense microbial cell-wall and surface components; endosomal TLRs sense nucleic acids from ingested microbes. TLR signaling activates NF-κB, which switches on inflammatory cytokine genes.
| TLR | PAMP recognized | Typical source |
|---|---|---|
| TLR4 | LPS (endotoxin; CD14 is its co-receptor on macrophages) | Gram-negative bacteria |
| TLR5 | Flagellin | Flagellated bacteria |
| TLR3 | Double-stranded RNA (endosomal) | Viruses |
| TLR9 | Unmethylated CpG DNA (endosomal) | Bacteria and DNA viruses |
Macrophage cytokines set the acute response.
- IL-1, IL-6, TNF-α — fever and the acute-phase response (IL-6 drives hepatic acute-phase proteins such as CRP); TNF-α also mediates septic shock.
- IL-8 — neutrophil chemotaxis.
- IL-12 — activates NK cells and drives Th1 differentiation, linking innate to adaptive immunity.
IL-8 recruits neutrophils: "clean up on aisle 8."
Complement. The humoral arm of innate immunity. C3b opsonizes microbes for phagocytosis; C3a and C5a are anaphylatoxins (C5a is also a potent neutrophil chemoattractant); and the membrane attack complex (C5b-9) lyses cells directly.
Bridging the two systems: antigen-presenting cells
Professional APCs. Dendritic cells (the professional APC), macrophages, and B cells present antigen on MHC class II to CD4 T cells. Dendritic cells are the key link: they capture antigen in tissue, carry it to the draining lymph node, and are the cells most able to prime naive T cells.
- PAMP detectedTLR on a dendritic cell in the tissue
- DC maturesUpregulates MHC II and costimulatory B7; migrates to lymph node
- Presents on MHC IITo a naive CD4 T cell (with B7–CD28 costimulation)
- T cell activatedAdaptive response begins
Innate signals decide whether adaptive immunity starts at all: antigen presented without costimulation from an activated APC tends to produce tolerance rather than activation.
NK cells
Innate lymphocytes. NK cells are lymphocytes of the innate system (markers CD16 and CD56) that kill virus-infected and tumor cells without prior sensitization, using perforin to open pores and granzymes to trigger apoptosis.
Missing self. NK cells preferentially kill cells that LACK MHC class I. Normal MHC I engages NK inhibitory receptors; its loss removes the brake. This is the mirror image of CD8 T cells, which require MHC I to recognize their targets. Viruses and tumors that downregulate MHC I to hide from CD8 T cells thereby flag themselves for NK killing.
ADCC and cytokines. Through CD16, an Fc receptor for IgG, NK cells perform antibody-dependent cellular cytotoxicity (ADCC), killing IgG-coated target cells. NK activity is boosted by IL-2, IL-12, IFN-α, and IFN-β.
Primary vs secondary responses and memory
Primary response. On first exposure there is a lag of about 1–2 weeks while naive clones expand; IgM appears first, at lower titer and lower affinity.
Secondary response. Driven by memory B and T cells, the recall response is faster (days), reaches a much higher titer, and is IgG-predominant because memory B cells have already class-switched and undergone affinity maturation.
| Feature | Primary | Secondary |
|---|---|---|
| Lag | ~1–2 weeks | Days |
| Main isotype | IgM first | IgG (class-switched) |
| Titer and affinity | Lower | Much higher |
| Responding cells | Naive lymphocytes | Memory B and T cells |
Memory is why vaccines work. Immunization creates memory B and T cells that mount a rapid, high-affinity IgG recall response on re-exposure; boosters exploit the secondary response. Conjugate and live vaccines generate memory cells for a fast, protective secondary response, whereas plain polysaccharide antigens are T-independent and produce weak memory. Innate immunity has no classic antigen-specific memory, which is why a splinter never "remembers."