• Describe the steps of the inflammatory response.
• Explain the functions of cells and molecules in the innate immune response.

If pathogens breach the first-line barriers (like skin or mucus), the body activates the innate immune response — a rapid, non-specific defence. It uses immune cells and signalling molecules to destroy invaders and slow the spread of infection. Unlike the adaptive response, the innate system does not form memory, but it plays a crucial early role in defending the body and activating the next line of defence.

Key Terms

• Leukocyte = A white blood cell involved in defending the body against infection.
• Phagocyte = A type of leukocyte that can engulf and digest pathogens (a process called phagocytosis).

Inflammatory Response

The inflammatory response is triggered when body tissues are damaged or invaded by pathogens. It is a key part of the innate immune system and aims to eliminate the cause of injury, clear out dead cells, and begin tissue repair.

Steps in the Inflammatory Response:

1. Tissue damage or pathogen entry occurs.
2. Mast cells in the tissue degranulate and release histamine.
3. Histamine causes vasodilation (widening of blood vessels) and increased vascular permeability, allowing more blood and immune cells to enter the site.
4. Complement proteins and cytokines attract phagocytes like neutrophils and macrophages.
5. Phagocytes engulf and destroy pathogens (phagocytosis).
6. The area shows classic signs of inflammation:

• Redness (due to increased blood flow)
• Heat (from increased metabolism)
• Swelling (from fluid leaking into tissues)
• Pain (due to pressure and chemical signals)

7. The tissue begins to repair, and the infection is cleared.

Cellular Components of the Innate Immune System

Neutrophils

• Fast-acting phagocytes and first responders to infection.
• Found in high numbers in blood.
• Engulf and destroy bacteria or fungi using enzymes and reactive chemicals.
• Short-lived — die after killing invaders (form pus).

Macrophages

• Long-lived phagocytes found in tissues.
• Clean up pathogens, debris, and dead cells.
• Release cytokines to attract more immune cells.
• Present antigens to helper T cells using MHC II molecules, linking to adaptive immunity.

Dendritic Cells

• Phagocytic cells located at body surfaces (e.g., skin, lungs).
• Capture pathogens, travel to lymph nodes, and present antigens via MHC II.
• Help activate naïve T cells, bridging innate and adaptive responses.

Natural Killer (NK) Cells

• A type of lymphocyte that doesn't rely on specific antigens.
• Patrol tissues looking for cells with abnormal or missing MHC I markers, common in virus-infected or cancerous cells.
• Release cytotoxic molecules to induce apoptosis (programmed cell death).

Mast Cells

• Found in tissues, especially under the skin and around blood vessels.
• Degranulate to release histamine and other inflammatory chemicals.
• Cause vasodilation, increased vascular permeability, and recruitment of immune cells.
• Important for initiating inflammation.

Eosinophils

• Specialised for fighting parasites like worms.
• Release toxic granules to kill large invaders.
• Also involved in allergic reactions and can contribute to tissue damage.

Non-Cellular Components

Interferons

• Signalling proteins released by virus-infected cells.
• Alert nearby cells to prepare antiviral defences and slow viral replication.
• Activate immune cells like NK cells.

Complement Proteins

• A group of proteins in the blood that assist immune responses by:
• Opsonising (tagging) pathogens for phagocytes
• Forming membrane attack complexes (MACs) to burst pathogens
• Attracting phagocytes to the infection site

Fever

• Triggered by chemical signals (e.g. cytokines) acting on the brain.
• Raises the body’s temperature to inhibit pathogen growth and boost immune reactions.