Final answer:
Pathogens like bacteria and viruses possess various structures, including capsules, cell walls, fimbriae, toxins, and viral covers, that contain antigens. These antigens are associated with adhesion to host cells and are important for colonization, infectivity, and virulence.
Step-by-step explanation:
Pathogens such as bacteria and viruses possess various structures that contain antigens. In bacteria, antigens can be found in the capsules, cell walls, fimbriae (also known as pili), toxins, and other extracellular enzymes that they secrete. Similarly, viruses have antigens associated with their capsids, envelopes, and spike structures that facilitate attachment to host cells.
For example, bacterial fimbriae and flagella have proteins called adhesins that bind to specific receptors on host cells, allowing the bacteria to attach to surfaces. Glycocalyces, such as capsules and slime layers, provide bacteria with protection, aid in adhesion to surfaces, and assist in the formation of biofilms. In certain bacterial pathogens, fimbriae and capsules play a role in evading the immune system.
Viruses use glycoproteins to attach to host cells through viral receptors. These receptors, which serve other purposes in the cell, enable viruses to recognize and infect specific cell types. The attachment is crucial for the subsequent penetration of the cell membrane, injection of the viral genome, and completion of replication inside the host cell.