01 Cellular and non cellular pathogens 2025

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What is a pathogen?

A disease-causing organism or agent, such as a virus, bacteria, fungus, parasite, or prion.

What is an antigen?

A molecule (usually a protein) that is recognised by the immune system and triggers an immune response.

What is a self-antigen?

A molecule on the surface of the body’s own cells that is recognised as “self” by the immune system.

What is a non-self antigen?

A molecule from outside the body that is recognised as “foreign” and triggers an immune response.

What is an allergen?

A harmless non-self antigen that causes an allergic immune response in some individuals.

What is a cellular pathogen?

A living pathogen made of cells. Examples: bacteria, protozoa, fungi, parasites.

What is a non-cellular pathogen?

A non-living infectious agent not made of cells. Examples: viruses and prions.

Compare cellular and non-cellular pathogens

Cellular pathogens are living and made of cells (e.g. bacteria); non-cellular pathogens are non-living (e.g. viruses, prions).

How do bacteria cause disease?

By reproducing quickly and releasing toxins that damage host tissues.

How do viruses cause disease?

By hijacking host cells to reproduce, then destroying them.

How do fungi cause disease?

By invading body tissues and disrupting normal function, often on skin or mucous membranes.

How do parasites cause disease?

By feeding on or living in the host, taking nutrients and damaging tissues.

How do prions cause disease?

By misfolding normal proteins in the brain, causing neurodegeneration.

Example of a bacterial pathogen

Mycobacterium tuberculosis – causes tuberculosis

Example of a viral pathogen

HIV – causes AIDS

Example of a fungal pathogen

Tinea – causes athlete’s foot

Example of a parasitic pathogen

Plasmodium – causes malaria

Example of a prion

Misfolded PrP protein – causes Creutzfeldt-Jakob disease (CJD)

What are MHC markers?

Proteins on cell surfaces that help immune cells distinguish self from non-self.

What type of immune response do allergens trigger?

An inappropriate immune response (hypersensitivity), often involving IgE and histamine release.