General Characteristics of Innate Immunity
The immune system provides two main forms of defense: non-specific and specific. The non-specific immune response represents the body's baseline line of defense. Unlike adaptive immunity, which protects against a specific pathogen and engages only when non-specific responses are insufficient, innate immunity possesses several unique features:
- High Speed. Mechanisms engage instantaneously at the earliest stages of pathogen invasion.
- Autonomy. The system functions fully even in the absence of a specific immune response.
- Lack of Memory. Innate immunity does not form 'long-term memory' following infection contact.
- Broad Spectrum of Targets. Attacks target not only microorganisms themselves but also host cells if they are infected by a pathogen or exhibit altered surface proteins.
First Line of Defense: Barriers and Fluids
The non-specific response comprises barrier, humoral, and cellular mechanisms.
Barrier Mechanisms Normal skin and intact mucosal epithelia serve as physical barriers preventing pathogen invasion and facilitating their expulsion. The body actively cleanses these barriers via:
- Mechanical organ movements: coughing, sneezing, and vomiting rapidly expel foreign agents.
- Mucociliary escalator: continuous directional movement of mucus in the respiratory tract driven by the rhythmic beating of ciliated epithelium.
- Peristalsis: ensures regular cleansing of the intestinal tract.
- Fluid flow: continuous flow of sterile urine flushes the urinary tract, while tear flow cleanses the ocular surface.
Humoral Mechanisms Body fluids are saturated with factors that kill microbes or inhibit their growth:
- Sweat possesses intrinsic antimicrobial properties.
- Lysozyme and polyamines are present in blood, tears, saliva, and intestinal secretions.
- Blood plasma components include the complement system, C-reactive protein (CRP), and interferon (IFN).
- Gastrointestinal secretory products (gastric juice, pancreatic enzymes, bile salts) create a chemically hostile environment for pathogen survival.
Cellular Response and the Phagocytic System
Various leukocyte populations participate in the non-specific immune response: polymorphonuclear leukocytes (including neutrophilic, basophilic, and eosinophilic granulocytes), mast cells, and natural killer (NK) cells.
A special place is occupied by the mononuclear phagocyte system (MPS). These cells are widely distributed in tissues and capable of phagocytosis—the engulfment and destruction of pathogens as well as defective host cells.
Macrophages historically derive their names from their tissue localization:
- Liver: stellate macrophages (Kupffer cells).
- Lungs: alveolar macrophages.
- Brain: microglial cells.
- Renal glomeruli: mesangial cells.
- Connective tissue and lymphoid system: histiocytes.
- Other tissues: macrophages.
Recognition Principle: What is a PAMP
Innate immunity is non-specific, yet it must flawlessly distinguish healthy autologous tissues from foreign agents. To achieve this, the system reacts to pathogen-associated molecular patterns (PAMPs). These are molecular structures common to all pathogens (e.g., specific bacterial cell wall molecules).
Properties of PAMPs:
- Origin Specificity. They are synthesized exclusively by microorganisms and are entirely absent in animal cells. Recognizing PAMPs serves as an unambiguous signal to attack a 'non-self' antigen.
- Group Character. These structures characterize entire groups of pathogens rather than a single specific species.
- Conservatism. PAMPs are vital for bacterial viability. Consequently, they evolve very slowly, allowing the immune system sufficient time to 'fine-tune' their efficient recognition.
Note: If the first line of defense—consisting of macrophages and granulocytes—is breached, adaptive (specific) immunity is engaged.