Cross-immunity is the ability of an immune response to one pathogen to provide partial or complete protection against a second, related pathogen due to shared antigenic determinants. This can arise from antibodies or T cells that recognize homologous epitopes and can neutralize or eliminate the related organism.
Explanation
Cross-immunity arises from the adaptive immune system’s capacity to recognize structural similarities among different microbes. When a host is exposed to a pathogen, it generates antibodies and T cells that target specific epitopes on that organism. If another microorganism shares homologous epitopes, these immune effectors can cross-react and provide some level of protection. This phenomenon is central to heterologous immunity and influences how populations respond to emerging infections. Cross-protection can be beneficial by reducing disease severity, but it can also lead to original antigenic sin, where prior exposure biases the immune response toward an outdated epitope. Cross-reactive memory T cells and antibodies are important in shaping responses to influenza viruses and coronaviruses, where antigenic drift and shift continually create new strains. Vaccine design often seeks to exploit cross-immunity by targeting conserved antigens to confer broad protection. Understanding cross-reactivity also helps explain why exposure to environmental bacteria or vaccines such as Bacillus Calmette–Guérin (BCG) can modulate immunity to unrelated pathogens and may contribute to non-specific protective effects.
Examples of cross-protection
Historical use of cowpox inoculation to protect against smallpox is a classic example of cross-immunity; vaccinia virus induces antibodies that neutralize variola virus because they share surface proteins. Seasonal influenza infection can confer some temporary immunity against related strains through cross-reactive antibodies and T cells, although antigenic drift reduces this effect over time. Exposure to common cold coronaviruses generates T-cell memory that can recognize SARS-CoV-2, which may influence disease severity. Dengue virus infection illustrates the complexity of cross-immunity; antibodies against one serotype can enhance infection with another serotype through antibody-dependent enhancement. BCG vaccination has been associated with decreased severity of unrelated infections, possibly via training of innate immunity and cross-reactive T cells.
The term cross-immunity underscores the interconnected nature of immune responses against related pathogens. It plays a role in disease ecology and vaccine development, offering insights into how prior exposures influence susceptibility and outcomes during outbreaks.
Related Terms: Cross-reactivity, Heterologous immunity, Antigenic drift, Vaccine design, Memory T cells