Biology VCE AOS 4 U1 - pathogens

Year 12 - 2026

Pathogens

Pathogens are agents that cause disease

Main groups of pathogens

Bacteria, fungi, parasites, protozoans, viruses, prions

Bacteria

Bacteria have a cell wall, plasma membrane, cytoplasm, ribosomes, and circular DNA

Fungi

Fungi have thread like strands called hyphar and cell walls of chitlin
they spread in spores

Protozoans

• Protozoa are diverse, they have membrane bound organelles and lack a cell wall

• Both asexual (binary fission and budding) and sxual
  

Parasites

Worms are long, soft and flattened with a basic nervous and digestive system
Sexual and asexual

Viruses

Viruses have a protein coat enclosing genetic matieral (DNA or RNA)
Enters cells and uses the host to make additional copies of themselves

Prions

Prions become infectious when a protein misfolds and a-helices beacomes a b-helices
misfolded prions convert normal prions into more misfolded prions

Categorising pathogens

Pathogens can be categorised as
- Cellular: has a cellular structure and living organisms
- Non - cellular: do not have a cellular structure and are non-living

Extracellular pathogens

Extracellular dont enter cells but interfere with the functioning of an organism

Intracellular pathogens

Will invade and destroy cells

Antigens

Antigens are the part of the pathogen that interacts with the immune system and include, surface proteins, DNA, RNA or free-floating molecules

Self vs non self

Two different types of antigens
Self

• Self

  • Any molecule located on the surface of cells that identifies it as self so the immune system dosent attack it

• non-self

  • Any molecule that the immune system reads as not belonging to that individual and therefore will stimulate an immune response

Self antigens

In humans the most important self-antigens take the form of major hiscompatibility complex (MHC) proteins, which can be divided to two classes

• MHC I

  • Are expressed on all nucleauted cells in the body, but not on cells without a nucleus (e.g. red blood cells

• MHC 2

  • Are found on a specialised cell of the immune sytem

how do MHC I proteins differ?

MHC I proteins found in our cells differ between individuals, MHC I proteins expressed on a donor organ will be different to the MHC I proteins of the organ receiver

Malfunctions involving antigens

There are two main types of antigens

• Auto immune

  • The immune system recognises self antigens as non-self (E.g arthiritis, multiple sclerosis)

• Allergic reactions

  • An overreaction to the prescence of an allergen

  • The immune system recognises these as non-self but initiates a very strong response