Pathogens can enter through wounds or respiratory, digestive and reproductive surfaces. Some use specific attachment molecules that bind compatible host-cell receptors. This helps determine which cells they can infect. Viruses enter susceptible cells and use cellular resources to reproduce; not every pathogen must live inside a host cell.
An antigen is a molecular feature recognised by immune receptors or antibodies. Antigens can be self or non-self; recognition involves molecular fit. Major histocompatibility complex, or MHC, molecules display peptide fragments at cell surfaces for T-cell recognition. MHC presentation is different from an antibody binding a free antigen. Recognition and appropriate immune signals together help coordinate a response rather than every contact producing the same outcome.
Example 1
A virus attaches only to cells with receptor R. Which cells are initially susceptible in this simplified model?
Cells displaying a compatible receptor R can bind the virus; attachment is a requirement in the stated model.
Example 2
Identify the role of a cell-surface MHC molecule displaying a peptide.
It presents a peptide fragment for recognition by an appropriate T-cell receptor.
Watch out:Viruses depend on host cells, but many bacterial and other pathogens can grow outside host cells.
Host entry and antigen recognition practice | Year 11 Biology | Ascent Arcade