BIO 5.2 - Antigens, pathogens and allergens

Immune system

The body system that helps resist infection and disease through specialised cells and proteins

Antigens

Molecules, parts of molecules or cells that can elicit an immune response

Antigens are found:

• Expressed or presented on the surface of the plasma membrane of cells

• Floating freely in body fluids

Self-antigens

Antigens on cells that are recognised by self-receptors as being part of the body

Non-self antigens

Antigens that do not belong to the body’s own cells

MHC (major histocompatibility complex)

Receptor proteins found on the surface of cells that identify the cell as “self”

MHC-1

• Found on all nucleated body cells

• Presents anything made inside the cell to natural killer cells and cytotoxic T cells

• Allows cells to be recognised as self

MHC-2

• Found only on antigen presenting cells such as dendritic cells, macrophages and helper T cells

• Presents antigens that are engulfed by the APC

• Helps activate the adaptive immune response

Disease

A condition that impairs the proper function of the body or one of its parts

Infectious disease

A disease that is caused by a pathogen

Non-infectious disease

A disease brought about by genetics, nutrient deficiency or excess, or mental conditions

Primary pathogens

Can cause disease any time they are present

Opportunistic pathogens

Only cause disease when the host’s defences have been weakened. E.g. by stress, HIV or poor nutrition

Cellular pathogens

A disease-causing agents made up of cells that can reproduce independently without relying on a host organism

Examples of cellular pathogens

• Bacteria

• Protozoans/protists

• Fungi

• Oomycetes

• Worms

• Arthropods

Bacteria

• Prokaryotic cells that reproduce through binary fission

• The symptoms of bacterial diseases are caused by the destruction of cells by bacterial enzymes, irritation by bacterial waste products and reactions to bacterial toxins

Protists/protozoans

• Unicellular eukaryotes

• Move between different hosts, spreading infection through ingestion of cysts, sexual transmission and insect vectors

Fungus

• Eukaryotic, multicellular, heterotrophic organisms with cell walls made of chitin

• Obtain energy by decomposing dead and dying organisms and absorbing their nutrients

• Cause infection by releasing enzymes that break down tissues and eliciting an immune response

Oomycetes

• Motile cells with flagella and walls made of of cellulose

• Spores are carried in droplets of water and invade plant tissues, releasing digestive enzymes that break down molecules. They suppress the host’s innate immune response and inhibit apoptosis

Worms

Cause swelling and can suppress the host’s immune response

Arthropods

• Invertebrates with exoskeletons, such as ticks, mosquitoes and lice

• Act as vectors that transmit pathogens to other organisms

Non-cellular pathogenic agents (3)

• Viruses

• Viroids

• Prions

Viruses

• Acellular agents made up of RNA or DNA enclosed in a protein coat

• Viruses recognise their specific host cells by binding to receptors on the host cell’s membrane

• The virus inserts its nucleic acid into the host cell, causing the host cell to reproduce the virus

• New viral particles are released by rupturing the host cell or budding

Viroids

• Tiny circular single stranded RNA molecules with no protein coat

• Infect host cells and use them to reproduce

• Only infect plants

Prions

• Abnormal proteins that replicate inside of host cells

• When a prion protein comes in contact with a normal protein, the normal protein becomes the abnormal prion protein