Cellular components of the first line
Innate immunity relies on several cell populations, each strictly specialized in fighting infection. The bulk of effectors are myeloid cells.
The myeloid lineage includes neutrophils, monocytes, and macrophages, whose primary task is to capture and digest foreign particles—a process that ends with intracellular killing. Additionally, neutrophils paired with eosinophils can kill pathogens from a distance via extracellular killing mechanisms.
The second important group is lymphoid cells. These include natural killer (NK) cells and NKT cells, which specialize in direct contact-dependent destruction of infected targets. A special population of γδ T cells also participates in early-stage defense.
Interestingly, protective properties are not restricted to classical leukocytes. Upon proper activation, epithelial and other tissue cells can acquire many functions of myeloid elements, including full phagocytic capability.
Humoral factors of the innate response
Cells do not work in isolation—they are constantly supported by soluble (humoral) substances that ensure communication and direct destruction of microbes.
- Myeloid cell factors: primarily pro-inflammatory cytokines and defensins.
- Lymphoid lineage factors: plasmacytoid dendritic cells actively produce type I interferons, while B1 lymphocytes generate natural antibodies.
- Tissue and plasma factors: the complement system plays a crucial role, providing cytolysis (destruction of microbial membranes) and extracellular opsonization. Acute-phase proteins, which are also activated during inflammation, serve as additional opsonins.
Dynamics of the local defense reaction
When a pathogen enters tissue, resident macrophages are the first to encounter it. However, their initial number is critically small to overcome the infection—this pool is only sufficient to trigger the inflammatory reaction. Full-scale local defense is only possible due to a massive influx of fresh leukocytes from the blood.
Cell migration into the focus of inflammation follows a strict sequence:
- Neutrophils. They are the most mobile elements and arrive at the lesion site first. They instantly deploy their high defensive potential through intensive phagocytosis.
- Monocytes. They migrate from the bloodstream following the neutrophil echelon. Once in the tissues, they differentiate into mature macrophages. Their functions in the focus are much broader: besides phagocytosis, they regulate inflammation, stimulate the proliferation of other cells, and initiate reparative processes in damaged tissue.
Concurrently with cellular infiltration, the humoral response unfolds: the complement system is triggered, the synthesis of acute-phase proteins begins, and bactericidal substances are released.
Special lymphoid populations and viruses
Innate immunity mechanisms can adapt. For example, during a viral infection, plasmacytoid dendritic cells and natural killer cells act as the main sensors, leading to their rapid and powerful activation.
In addition to classical participants, specific lymphocytes are actively recruited into the first line of defense:
- B1 lymphocytes. They synthesize natural antibodies that are polyspecific and have low affinity for the antigen. Interacting with the pathogen, these antibodies activate the complement cascade, which then attracts new phagocytes to the focus.
- NKT cells. They play the role of a crucial early source of interferon gamma (IFN-γ). This cytokine is vital for enhancing macrophage activation.
Transition to the adaptive response
All the cellular and humoral factors listed above pursue one main goal: to ensure the elimination (complete destruction and removal) of the pathogen from the body.
In most situations, this is sufficient for recovery. However, in cases where the first line of defense fails and elimination does not occur, the system moves to the next level. The second line of defense is triggered—the adaptive immune response.
Briefly: answers to topical questions
Which receptors ensure pathogen recognition by innate immunity cells?
Pathogen recognition by innate immunity cells is mediated by pattern recognition receptors that bind pathogen-associated molecular patterns (PAMPs). The following groups of receptors are distinguished:
- Membrane receptors — Toll-like receptors (TLR 1–11), C-type lectins, scavenger receptors, and integrins.
- Intracellular receptors — NOD-like receptors (NLRs), RIG-like receptors (RLRs), and DAI.
- Soluble (secreted) receptors — pentraxins, collectins, complement system components, and ficolins.
Which specific pro-inflammatory cytokines are secreted by myeloid cells?
Myeloid cells (activated monocytes, macrophages, neutrophils, and dendritic cells) secrete pro-inflammatory cytokines:
- Tumor necrosis factor alpha (TNFα / TNF-α).
- Interleukin-1 (IL-1, including IL-1β).
- Interleukin-6 (IL-6), which possesses pro- and anti-inflammatory properties.
- Interleukin-18 (IL-18).
- Interleukin-17 (IL-17) — produced, in particular, by neutrophils.
Through what mechanisms is intracellular killing carried out in phagocytes?
Intracellular killing in phagocytes is carried out within phagolysosomes via oxygen-dependent and oxygen-independent mechanisms. The main bactericidal factors include:
- Reactive oxygen species — formed as a result of the respiratory burst (superoxide, hydrogen peroxide, hydroxyl radical, singlet oxygen).
- Reactive nitrogen species — nitric oxide and its metabolites.
- Acidification of the medium — decrease in pH to 4.5–5.0 due to V-ATPase activity.
- Cationic proteins — serprocidins, lysozyme, lactoferrin.
- Acid hydrolases — myeloperoxidase, β-glucuronidase, and other enzymes.
- Bactericidal peptides — alpha and beta defensins, cathelicidins.
Which specific proteins belong to the acute-phase proteins of inflammation?
Acute-phase proteins of inflammation (acute-phase reactants) include specific proteins synthesized by hepatocytes, macrophages, and other cells. This group includes:
- C-reactive protein (CRP) — belongs to the pentraxin group, activates the cytolytic activity of natural killers, and inhibits platelet aggregation.
- Fibrinogen — its level increases during the systemic inflammatory response.
- T-kininogen — a precursor of kinins and an inhibitor of α-cysteine proteinases.
- Apoferritin — stimulates the production of superoxide bactericidal ions by leukocytes.
- Pentraxins — a group of proteins including short and long pentraxins.
Remembering the migration order is simple: Neutrophils — Beginning (arrive first, fast), Monocytes — Macrophages (arrive second, clean up and restore tissue).
Class and exam questions
Are tissue macrophages sufficient to suppress the infection?
No, the initial number of macrophages and other effector cells at the site of invasion is only sufficient to trigger the reaction. Real defense requires an influx of leukocytes from the blood.
Are epithelial cells capable of destroying microbes?
Yes, under the influence of activation, they can acquire many defense functions characteristic of myeloid cells, including the capacity for phagocytosis.
What is the specific feature of first-line defense antibodies?
Natural antibodies produced by B1 cells are polyspecific and have low antigen affinity. Their main task is to bind the pathogen to activate the complement system.
When is the second line of immunity triggered?
The adaptive immune response is engaged when first-line mechanisms (innate immunity) fail to ensure complete pathogen elimination.
What else to study on this topic
- Mechanisms of intracellular and extracellular killing by myeloid cells
- Complement activation pathways and extracellular opsonization
- The role of interferon gamma (IFN-γ) in macrophage activation
- Structure and functions of low-affinity natural antibodies of B1 lymphocytes
- Specificity of virus recognition by plasmacytoid dendritic cells
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