Intracellular Sensors: NOD- and RIG-Like Receptors
These receptors are located directly within the cell cytoplasm. Their main task is to monitor the internal space in case a pathogen manages to breach the outer membrane barriers.
NOD-Like Receptors (NLRs)
This family of endogenous receptors contains a characteristic NOD domain—a specialized oligomerization segment capable of binding nucleotides. NLRs act as an emergency alert system for bacterial agents entering the cell.
They exhibit distinct ligand specificity (for bacterial components):
- NOD1 recognizes muramyl tripeptide.
- NOD2 specializes in muramyl dipeptide.
How it works: When the receptor binds its ligand, the transcription factor NF-$\kappa$B is activated inside the cell. This triggers a potent response—the synthesis of pro-inflammatory cytokines and chemokines that recruit other immune cells to the infection site.
RIG-Like Receptors (RLRs)
Unlike NLRs, this group of cytoplasmic sensors targets viruses.
- Specificity: The main target of RLRs is viral genetic material, primarily double-stranded RNA, along with certain other molecules.
- Activation Effect: Upon detecting a virus, these receptors initiate the production of protective molecules. First, interferons $\alpha$ and $\beta$ are produced to provide antiviral defense. Second, classic pro-inflammatory cytokines (such as IL-6 and TNF-$\alpha$) are released.
Surface Trappers: The Mannose Receptor
To catch microorganisms from the outside, cells utilize surface structures. A classic example is the mannose receptor, which belongs to the C-type lectin receptor family.
- Structure and Localization: It contains specific lectin domains designed by nature to tightly bind carbohydrates. This receptor is located on the surface of mononuclear phagocytes.
- Target: The ideal "prey" for this receptor consists of microorganisms with cell walls rich in mannose structures.
- Function: The primary role of the mannose receptor is the physical binding of microbes. Once contact is established, the cell initiates endocytosis to internalize the bound pathogen for subsequent digestion.
Cellular Cleaners: Scavenger Receptors
The innate immune system not only fights external enemies but also maintains internal homeostasis. This is managed by scavenger receptors, which are predominantly expressed on macrophages and dendritic cells (and to a lesser extent on epithelial cells).
Their primary function is housekeeping. They capture and target various modified molecules and dead cells for endocytosis, clearing the body's internal environment.
What scavengers recognize (ligands):
- Cellular degradation products (e.g., breakdown products of sialic acids).
- Low-density lipoproteins (LDLs) and various oxidized lipids.
- Bacterial cell wall components (such as lipoteichoic acid).
- Apoptotic bodies (fragments of cells that underwent programmed cell death).
- The body's own aging cells (a typical example being senescent erythrocytes).