Learning Objective: By the end of this lesson, medical students should be able to differentiate between innate and adaptive immunity in terms of components, mechanisms, response characteristics, and key molecular features — and apply this understanding to clinical immunopathology questions on the USMLE Step 1.
Overview
| Feature | Innate Immunity | Adaptive Immunity |
|---|---|---|
| Main Components | Neutrophils, macrophages, monocytes, dendritic cells, NK cells (lymphoid origin), complement, epithelial barriers, secreted enzymes | T cells, B cells, circulating antibodies |
| Mechanism of Action | Germline encoded; recognizes common microbial patterns | Generates diversity via V(D)J recombination during lymphocyte development |
| Response to Pathogens | Nonspecific; rapid (minutes to hours); no memory response | Highly specific; develops over days; memory response is faster and stronger |
| Secreted Proteins | Lysozyme, complement, C-reactive protein (CRP), defensins, cytokines | Immunoglobulins, cytokines |
| Recognition Mechanism | Pattern Recognition Receptors (PRRs) such as Toll-like receptors (TLRs) recognize PAMPs and DAMPs → activation of NF-κB → release of pro-inflammatory cytokines | Memory B and T cells recognize previously encountered antigens → robust, rapid response |
| Examples of PAMPs | Lipopolysaccharide (Gram-negative bacteria), flagellin (bacteria), viral nucleic acids | N/A |
| Examples of DAMPs | Mitochondrial DNA, histones, heat shock proteins | N/A |
| Age-Related Change | Generally preserved | Decreases with age (immunosenescence) |
Key Points
- Innate Immunity = Immediate and non-specific
- Recognizes patterns, not individual antigens.
- Involves macrophages, neutrophils, NK cells, complement.
- No memory formation.
- Adaptive Immunity = Specific and acquired
- Involves T and B lymphocytes.
- Undergoes V(D)J recombination for antigen receptor diversity.
- Generates immunologic memory → faster response upon re-exposure.
- Clinical Relevance:
- TLR defects → recurrent bacterial infections.
- NK cell deficiency → increased viral infections.
- Immunosenescence contributes to poor vaccine response in elderly.
