pathogen
Summary
A pathogen is an infectious agent, such as a bacterium, virus, fungus, protozoan, or parasitic worm, that causes disease in a host. The MCAT tests whether you can identify the type of pathogen and predict which immune response or treatment applies, for example antibiotics for bacteria but not viruses.
Detail
Meaning: A pathogen is an agent that causes disease in a host. Main types are bacteria (prokaryotes), viruses (acellular, replicating only inside host cells), fungi, protozoa, and parasitic worms. Prions (misfolded proteins) are also infectious agents. Pathogens cause harm by damaging cells, releasing toxins, or triggering damaging immune responses.
On the MCAT: Passages ask you to match a pathogen to the immune response. Extracellular pathogens such as many bacteria are met by antibodies and phagocytes, while virus-infected cells are attacked by cytotoxic T cells that recognize viral peptides displayed on MHC I. The pathogen's surface antigens are what B and T cells recognize. Questions also use antibiotic targets (the bacterial cell wall, bacterial ribosomes), viral life cycles, and antigenic variation, which helps explain why a new flu vaccine is needed.
Common confusion: Pathogen is not the same as microbe. Many microbes, such as much of the gut flora, are harmless or helpful, and opportunistic pathogens cause disease mainly when host defenses are weakened. Exposure also does not guarantee disease. Antibiotics act on bacteria and do not work against viruses.
Sources
- Campbell Biology
- Microbiology: An Introduction
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.