The Body's Defence Mechanisms

The antiviral substance secreted by virus-infected cells that inhibits viral multiplication is called interferon.

The human body is constantly exposed to pathogens – bacteria, viruses, fungi, and parasites. To survive, the body has evolved a multi-layered defence system that acts even before a specific immune response develops. This non-specific, first-line protection is called innate immunity, and it operates through physical barriers, chemical barriers, cellular responses, and inflammation.

The skin and mucous membranes form the body's outermost, non-specific shield against pathogen entry.

1. Skin: an unbroken keratinised layer that physically blocks microbial entry
2. Mucous membranes: line the respiratory, digestive, and urogenital tracts and trap pathogens in mucus
3. Cilia: hair-like projections in the respiratory tract that sweep trapped microbes and mucus outward
4. Tears and saliva: continuously wash away surface pathogens

Phagocytosis

• Carried out mainly by macrophages and neutrophils
• These cells engulf and digest pathogens through enzymatic breakdown
• Rapid, non-specific, and does not require prior exposure to the pathogen

Interferons

• An antiviral substance secreted by virus-infected cells that inhibits viral multiplication in neighbouring healthy cells.
• Interferons act as an early-warning chemical signal, alerting nearby cells to resist viral replication
• Important in limiting the spread of viral infections before antibodies are produced

A-E. Steps in phagocytosis of a bacterium by a neutrophil

Inflammation is the body's localised or systemic reaction to tissue damage or infection.

• Histamine and serotonin are released, increasing blood vessel permeability
• Increased blood flow brings more phagocytic cells to the infection site
• Local inflammation: swelling, redness, heat, and pain confined to the injury site
• Systemic inflammation: a body-wide response, often accompanied by fever
• Pyrogens: chemical substances that reset the body's temperature control centre, producing fever, which slows pathogen reproduction

• Innate immunity provides the body's initial, non-specific protection against pathogens using a multi-layered defence system.
• Physical barriers, such as skin and mucous membranes, prevent microbial entry, while chemical barriers, such as stomach acid and lysozyme, actively destroy trapped pathogens.
• During cellular responses, specialised cells such as neutrophils and macrophages engulf and enzymatically break down pathogens through a process known as phagocytosis.
• When infected by a virus, cells secrete an antiviral substance called interferon to alert nearby healthy cells and inhibit further viral multiplication.
• The inflammatory response helps isolate and resolve infections by increasing local blood flow to deliver defensive cells or by inducing a systemic fever to slow pathogen reproduction.