Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 43.1 In Innate Immunity, Recognition and Response Rely on Traits Common to Groups of Pathogens
Innate immunity is the first line of defense found in all animals. It involves rapid recognition and general responses to common features of pathogens, using barrier defenses, cellular responses, and chemical signals to eliminate invaders before infection spreads.
1. Innate Immunity in Invertebrates
- Insects rely on barrier defenses like the chitin exoskeleton and gut lining
- Lysozyme in digestive secretions breaks down bacterial cell walls
- Hemocytes carry out phagocytosis and secrete antimicrobial peptides
- Recognition proteins bind broadly shared microbial features, activating immune responses
- In response to fungi, Toll receptors activate pathways that lead to fungal-specific peptides
- Viruses with double-stranded RNA trigger intracellular defenses using Dicer-2 and Argo enzymes
2. Barrier Defenses in Vertebrates
- Skin and mucous membranes prevent pathogen entry
- Mucus traps microbes; cilia sweep them away
- Secretions like saliva, tears, and sweat inhibit growth with lysozymes and low pH
- Stomach acid kills many ingested pathogens
3. Cellular Innate Defenses
- Phagocytes (e.g., macrophages, neutrophils) engulf and destroy microbes
- Use Toll-like receptors (TLRs) to detect bacterial or viral components
- Dendritic cells trigger adaptive immunity
- Eosinophils combat parasites with enzymes
- Natural killer cells detect abnormal cells and trigger cell death without phagocytosis
4. Inflammatory Response
- Macrophages and mast cells release signals (e.g., cytokines, histamine)
- Histamine causes blood vessels to dilate and become leaky
- Neutrophils and antimicrobial peptides enter tissue
- Accumulated fluid, WBCs, and debris form pus
- Can be localized or systemic (fever, high WBC count)
- Severe systemic responses (e.g. septic shock) can be fatal
- Chronic inflammation (e.g. Crohn’s disease) disrupts normal function
5. Lymphatic System
- Lymph collects fluid from tissues and transports it via lymphatic vessels
- Lymph nodes filter pathogens and activate immune cells
- Dendritic cells bring antigens to lymph nodes to stimulate adaptive immunity
6. Antimicrobial Proteins
- Interferons are secreted by virus-infected cells to block viral replication
- Other interferons activate macrophages
- Complement system: ~30 plasma proteins that lyse pathogens or enhance inflammation
7. Pathogen Evasion
- Some pathogens avoid destruction via capsules (e.g. Streptococcus pneumoniae)
- Others survive inside host cells (e.g. Mycobacterium tuberculosis), avoiding detection
In a Nutshell
Innate immunity provides immediate, general defense against a wide range of pathogens. It includes physical barriers, immune cells that detect conserved microbial features, and chemical responses like inflammation and antimicrobial proteins. Though not pathogen-specific, innate immunity is essential for early defense and for activating adaptive responses.