# Role of Protein Glycoslation in Host-Pathogen Interaction
## 2 Host-Pathogen Interactions  
Host-pathogen interactions (HPIs) are complex and dynamic in nature and crucial for our understanding of infectious diseases. Most studies on HPIs primarily focus on how pathogens attack and exploit hosts, e.g., how pathogen invades hosts, evades from host defense, and proliferates in  
Figure 1: Diagram showing the structure of \(N\)-linked and \(O\)-linked glycosylation types. **(A)**\(N\)-linked glycoprotein. The core oligosaccharides charide links to the amide nitrogen of asparagine (Asn) in the consensus sequence Asn-X-Ser/Thr (Ser, Thr, and X represent the serine, threonine and random amino acids). **(B)** Different subtypes of \(N\)-glycoprotein. High-mannose, oligosaccharides charide chains linked to the core oligosaccharides charide via mannose; Hybrid, oligosaccharide chains linked to the core oligosaccharides charide via GlcNAc and mannose; Complex, oligosaccharides charide chains linked to the core oligosaccharides charide via GlcNAc. **(C)**\(O\)-linked glycoprotein, oligosaccharides are bonded to the hydroxyl of Ser, Thr, hydroxyl lysine (HyL) or hydroxylproline (HyP) of proteins. Although the \(O\)-linked glycoprotein has no core oligosaccharides charide, constant protein-glycans linkage was found in some types of proteins, such as GalNAc-Ser/Thr linkage found in Mucin-type proteins, \(\iota\)-fucose- or n-glucose- Ser/Thr linkage found in epidermal growth-factor-like repeats (EGF), n-mannose- Ser/Thr linkage found in Cadherin, dystoglycan or fungi and prokaryotes glycoproteins (Cadherin/dystrophycan/O-Man), GlcNAc-Ser/Thr linkage found in nucleocytosol glycoproteins or secreted glycoproteins in eukaryotic cells except yeast (Cytoplasm/secreted protein), n-galactose- or l-arabinose- HyP linkage found in hydroxyproline-rich glycoproteins (HRGPs) and n-galactose- HyL linkage found in Collagen. hosts, and in turn how host protects themselves against pathogens, e.g., how host immune responds to invaders [52]. Pathogen invasion starts from penetration into the host. Several ways of entry or pathways can be used, such as entering or (and) adhering to wounds on the host (cells) surface, mucosa membrane or natural orifices [53; 54]. After entering hosts, pathogens interact with host cells for their survival and proliferation, with different targets and in specific mechanisms. For instance, viral pathogens invade host cells by merging their envelope with the membrane of host cells [55]; parasitic nematodes, fungi or bacteria can use special secretion systems to release effectors into host cells, manipulating some physiological and biochemical processes in hosts [56; 57; 58]; fungi also can utilize effectors on their hyphae surface to target host cell receptors and induce the endocytosis or cellular uptake of pathogens [59]. Pathogens may also interact with the host immune system, including the triggering of host immune responses and the pathogen evasion from these. Host immune responses are stepwise, first triggered by pathogen conserved molecules recognized by host-specific receptors. Then, through signal pathways, the defense signals are transmitted to the out- or in-side of cells. Finally, hosts receive the defense signals and then activate defense responses to tackle pathogens. Pathogens might attack any defense steps, they can evade recognition by host receptors through changing or masking their conserved molecules [60; 61; 62; 63]. Invaders also may sabotage defense responses through disturbing the activity of key proteins or degrading signal molecules, which are related to a variety of defense responses or function in inhibiting the growth of pathogens or killing pathogens directly [59; 64; 65]. Proliferation is essential for a successful pathogen invasion. This process is normally accomplished by manipulating the host cellular machinery in different ways.