5.2 antigens, pathogens and allergens

antigen

molecules or parts of molecules that stimulate an immune response

triggered by receptors on the cell surface that bind in a specific manner to non-self molecules

difference between self and non-self antigens

self antigens: belong to the body’s own cells, recognised by self receptors

non-self antigens: do not belong to the body’s own cells, foreign, can be identified as invaders and be attacked by the immune system

difference between MHC-I and MHC-II self markers

MHC-I self markers:

• present on all nucleated cells of the body

• contains a a binding site for an antigen (allow cells to be recognised as ‘self’)

MHC-II self markers:

• present on specific white blood cells

  • antigen presenting cells (macrophages, dendritic cells)

  • helper T cells

• involved in the adaptive immune response

pathogen

agents that cause diseases in their hosts (can be cellular or noncellular)

difference between cellular and non-cellular pathogens

cellular pathogens:

• classified as living organisms

• able to reproduce independently

non-cellular pathogens:

• classified as non-living

• cannot reproduce without a host

• hijack the host’s processes in order to replicate

cellular pathogen examples

• bacteria

• protozoa

• fungi

• parasites

key features of bacteria

• cellular pathogen

• prokaryotic

• reproduce via binary fission

• can produce exotoxins and endotoxins

exotoxins

toxins that are secreted into the surrounding medium by a microorganism as it grows

endotoxins

toxic parts of the outer membrane of some Gram-negative bacteria that are released when the bacteria die

key features of protozoa

• cellular pathogen

• eukaryotic

• reproduce via binary fission

• transmission via ingestion of cysts, sexual transmission or insect vectors

key features of fungi

• cellular pathogen

• eukaryotic

• grow from the tips of filament (hyphae) that make up the bodies of the organism (mycelia) and digest organic matter externally before absorbing it

• reproduce via formation of spores

key features of parasites

• cellular pathogen

• an organism that lives in or on an organism of another species (host) and benefits by deriving nutrients at the host’s expense

  • viruses, helminth worms

non-cellular pathogen examples

• viruses

• viroids

• prions

key features of viruses

• non-cellular pathogen

• can only replicate within host cell (parasite)

• releases virus particles by lysis

key features of viroids

• non-cellular pathogen

• can only replicate within host cell (parasite)

• simpler form of viruses

key features of prions

• non-cellular pathogen

• made of protein, lacks nucleic acids

• transformed by contact with a harmful prion