General characteristics of innate immunity
The immune system provides two main forms of defense: nonspecific and specific. The nonspecific immune response represents the body's basic line of defense. Unlike adaptive immunity, which protects against a specific pathogen and is engaged only when the nonspecific response is insufficient, innate immunity possesses a number of unique features:
- High speed. Mechanisms are triggered instantly, at the very earliest stages of infection penetration.
- Autonomy. The system functions fully even in the absence of a specific immune response.
- Absence of memory. Innate immunity does not form "long-term memory" of contact with an infection.
- Wide spectrum of targets. Attack is directed not only at microorganisms themselves, but also at the host's own cells if they are infected by a pathogen or have altered proteins on their surface.
First line of defense: barriers and fluids
The nonspecific response includes barrier, humoral, and cellular mechanisms.
Barrier mechanisms Normal skin and intact mucosal epithelium serve as a physical barrier that prevents pathogen invasion and facilitates their expulsion. The body actively clears these barriers:
- Mechanical movement of organ walls: coughing, sneezing, and vomiting help rapidly expel foreign agents.
- Mucociliary escalator: mucus is constantly propelled in the respiratory tract through the rhythmic beating of ciliated epithelial cilia.
- Peristalsis: ensures regular cleansing of the intestine.
- Fluid flow: a continuous flow of sterile urine flushes the urinary tract, and the flow of tears cleanses the ocular surface.
Humoral mechanisms The body's biological fluids are saturated with factors that kill microbes or inhibit their growth:
- Sweat possesses intrinsic antimicrobial properties.
- Lysozyme and polyamines are found in blood, tear fluid, saliva, and intestinal secretions.
- Blood plasma components include the complement system, C-reactive protein (CRP), and interferon (IFN).
- Secretory products of the gastrointestinal tract (gastric juice, pancreatic enzymes, bile salts) create a chemically hostile environment for pathogen survival.
Cellular response and the phagocyte system
Various cell populations participate in the nonspecific immune response: polymorphonuclear leukocytes (which include neutrophil, basophil, and eosinophil granulocytes), mast cells, and natural killer cells (so-called NK cells).
A special place is occupied by the mononuclear phagocyte system (MPS). These are cells widely distributed in tissues and capable of phagocytosis—the engulfment and destruction of pathogens, as well as the body's own defective cells.
Macrophage names historically depend on their tissue localization:
- Liver: stellate endothelial cells (Kupffer cells).
- Lungs: alveolar macrophages.
- Brain: microglial cells.
- Renal glomeruli: mesangial cells.
- Connective tissue and lymphoid system: histiocytes.
- Other tissues: macrophages.
Recognition principle: what are PAMPs
Innate immunity is nonspecific, but it must unerringly distinguish healthy self tissues from foreign agents. To achieve this, the system reacts to pathogen-associated molecular patterns (pathogen-associated molecular patterns — PAMPs). These are molecular structures common to all pathogens (e.g., specific bacterial cell membrane molecules).
Properties of PAMPs:
- Specificity of origin. They are synthesized exclusively by microorganisms and are completely absent in animal cells. PAMP recognition serves as an unambiguous signal to attack the "non-self" antigen.
- Group character. These structures are characteristic not of a single specific species, but of an entire range of pathogens.
- Conservatism. PAMPs are vital for the bacterium's survival. For this reason, they evolve very slowly, giving the immune system time to "tune" its effective recognition.
Note: If the first line of defense, consisting of macrophages and granulocytes, is breached, specific (adaptive) immunity comes into play.
Briefly: answers to topical questions
Which humoral factors are part of the innate immune system?
The innate immune system includes humoral factors of biological fluids that kill pathogens or inhibit their growth. These include:
- Sweat — possesses antimicrobial properties.
- Lysozyme and polyamines — found in blood, tear fluid, saliva, and intestinal secretions.
- Blood plasma components — the complement system and C-reactive protein.
- Interferon.
- GI secretory products — gastric juice, pancreatic enzymes, and bile salts.
Which receptors on innate immune cells recognize PAMPs?
Pathogen-associated molecular patterns (PAMPs) are recognized by pattern recognition receptors (PRRs). They are subdivided into the following groups:
- Secreted receptors — circulate in biological fluids (blood, lymph).
- Membrane-bound receptors — TLRs (Toll-like receptors) and CLRs (C-type lectin receptors) located on the plasmalemma and in endosomal membranes.
- Cytosolic receptors — intracellular sensors, including NOD1 and NOD2 (recognize bacterial peptidoglycans), RIG-I and MDA5 (viral nucleic acid sensors), DAI, and inflammasomes.
- Scavenger receptors — receptors that recognize bacterial lipid components.
What are the main differences between innate and adaptive immunity?
The main differences lie in specificity, reaction speed, and recognition mechanisms.
| Characteristic | Innate immunity | Adaptive immunity |
|---|---|---|
| Recognition target | Conservative molecular structures | Antigenic epitopes |
| Self-nonself discrimination | Perfect, developed in phylogenesis | Imperfect, formed during ontogenesis |
| Need for costimulation | No | Present |
| Time to effect | Immediate | Takes time |
| Receptor gene formation | Genetically determined | Formed during cell differentiation |
| Cellular distribution | All cells in a population express identical receptors | Clonal |
| Cells carrying receptors | Any nucleated cells | Only B and T lymphocytes |
Which leukocyte populations participate in the cellular response of innate immunity?
Several main leukocyte populations participate in the cellular response of innate immunity. These include:
- Polymorphonuclear leukocytes — neutrophilic, basophilic, and eosinophilic granulocytes.
- Mononuclear phagocytes — blood monocytes and tissue macrophages (histiocytes, Kupffer cells, alveolar macrophages, microglial cells, mesangial cells).
- Mast cells (labrocytes) — involved in the release of inflammatory mediators and chemotaxis.
- Natural killer cells (NK cells) — large granular lymphocytes that destroy infected and tumor cells.
What are the functions of natural killer (NK) cells in the nonspecific immune response?
Natural killer (NK cells) provide a rapid response to foreign antigens and perform two main functions:
- Cytotoxic function — destruction of tumor cells, protozoa, as well as cells infected with viruses and bacteria. Lysis is carried out via a perforin-dependent mechanism: perforins form pores in the target membrane through which granzymes enter, triggering apoptosis.
- Regulatory function — coordination of the innate immune response through the production of cytokines, in particular interferon-gamma (IFN-γ).
Class and exam questions
What happens if innate immunity fails to cope with an infection?
If the first line of defense (macrophages and granulocytes) is breached, the body triggers a specific (adaptive) immune response aimed at destroying the specific pathogen.
Why does the immune system recognize PAMPs, and why don't bacteria get rid of them?
These molecular structures are critically important for the microorganisms' survival themselves. They evolve extremely slowly, so the immunity manages to set up their flawless recognition.
Do PAMP structures occur in healthy human cells?
No, these molecular patterns are synthesized exclusively by microorganisms and are completely absent in animal cells. Their appearance is a signal to attack a "non-self" antigen.
What else to study on this topic
- Mechanisms of the respiratory tract mucociliary escalator
- The role of the complement system and C-reactive protein in blood plasma
- Functions of natural killer (NK) cells and mast cells
- Interaction of polymorphonuclear leukocytes with pathogens
- The process of phagocytosis in the mononuclear phagocyte system
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