Innate Immunity

Frequently asked questions

What exactly do innate immune cells recognize upon encountering a pathogen?

Innate immune cells recognize not individual molecules, but general structural features—pathogenicity patterns and danger patterns.

• Pathogen-associated molecular patterns (PAMPs) — conservative microbial structures foreign to the host (lipopolysaccharide, peptidoglycan, flagellin, viral RNA).
• Damage-associated molecular patterns (DAMPs) — endogenous molecules released by host cells under stress, damage, or death (heat shock proteins, uric acid).

Recognition of these patterns allows the immune system to detect danger long before clinical manifestations of infection appear.

Through which receptors do innate immune cells recognize pathogenicity patterns?

Innate immune cells recognize pathogenicity patterns via pattern recognition receptors (PRRs).

Based on localization, PRRs are classified into:

• Membrane receptors — TLRs, C-type lectins, scavenger receptors, integrins.
• Intracellular / cytosolic receptors — NOD-like, RIG-like, DAI; the inflammasome is also considered a cytosolic sensor.
• Secreted receptors — pentraxins, collectins, complement system components, ficolins.

Functionally, PRRs ensure pathogen recognition, signal transduction initiation, and synthesis of effector molecules.

What are the pathways of complement system activation?

There are three main pathways of complement system activation:

• Classical pathway.
• Lectin pathway.
• Alternative pathway.

In general, the triggers for the complement system are the antigen-antibody complex and the microorganism itself. The lectin pathway can be initiated by the binding of mannose-binding lectin to microbial carbohydrates.

All three pathways converge at the stage of enzymatic complex formation—convertases. Convertases cleave component C3, and the ultimate goal of all pathways is the assembly of the membrane attack complex (MAC) on the target cell surface.

Which innate immune cells are professional phagocytes?

Innate immune cells involved in phagocytosis include:

• Monocytes.
• Macrophages.
• Neutrophils.
• Eosinophils.
• Basophils.

Macrophages perform phagocytosis and are potent producers of pro-inflammatory and regulatory cytokines. Neutrophils act as microphages: they phagocytose bacteria and small tissue debris.

Dendritic cells belong to the antigen-presenting cells of innate immunity; they capture antigen via pinocytosis, less commonly phagocytosis, and effectively activate naive T cells.

What stages comprise the process of phagocytosis?

Traditionally, 8 sequential stages of phagocytosis are distinguished:

• Chemotaxis — directed movement of the phagocyte toward the object.
• Adhesion — establishing contact and attachment of the phagocyte to the object.
• Membrane activation — preparation for engulfment.
• Immersion — enveloping the object.
• Phagosome formation — closure of the membrane and internalization.
• Phagolysosome fusion — fusion of the phagosome and lysosome.
• Killing and degradation — intracellular digestion of the object.
• Release of degradation products.

In a broader classification, these processes are grouped into 4 main stages: approach, recognition and adhesion, engulfment, and destruction.

Through what mechanisms does normal microflora provide barrier function?

Normal microflora provides barrier function through colonization resistance—the ability to prevent pathogen colonization due to antagonistic activity. Key defense mechanisms include:

• Competition — contesting nutrients with foreign flora due to high biological potential.
• Acid production — synthesis of lactic, acetic, and other short-chain fatty acids, as well as secondary bile acids.
• Synthesis of antimicrobial substances — production of antibiotic-like substances, bacteriocins, and microbial lysozyme.

Additionally, microflora performs an immunostimulatory function: its antigens continuously stimulate the immune system.

What is the main difference between innate and adaptive immunity?

Innate immunity acts instantaneously and targets general pathogen patterns, whereas adaptive immunity takes several days to develop, is highly specific to a particular antigen, and leaves immunological memory.

Which cells belong to the innate immune system?

Macrophages, monocytes, all granulocytes (neutrophils, basophils, eosinophils), dendritic and mast cells, as well as natural killer (NK) cells.

What are humoral factors of innate immunity?

These are soluble protective molecules: interferons, complement system proteins, acute-phase proteins, lysozyme, and surfactant proteins.