How does the target of a non-specific defence differ from that of a specific defence?

Non-specific defences target all pathogens generally without recognizing specific antigens. In contrast, specific defences identify and attack unique molecular markers on a particular strain of pathogen.

What is the functional difference between the role of skin and the role of phagocytes?

The skin acts as a primary physical barrier to prevent pathogens from entering the body at all. Phagocytes act as a secondary internal defence that seeks out and destroys pathogens that have already bypassed the external barriers.

Compare the nucleus of a phagocyte with that of a lymphocyte.

A phagocyte has a multi-lobed nucleus which allows it to change shape easily during engulfment. A lymphocyte typically has a large, round nucleus that occupies most of the cell's volume.

Why is it incorrect to say that fibrinogen forms the blood clot mesh?

Fibrinogen is a soluble protein and cannot form a solid structure. It must be converted into insoluble fibrin, which forms the actual physical fibers of the mesh that trap blood cells.

What is a common misconception regarding the role of stomach acid in digestion vs. immunity?

While stomach acid helps in digestion, its role in immunity is to act as a chemical barrier. It kills most ingested pathogens by denaturing their proteins before they can reach the intestines and cause infection.

Why is it a mistake to classify lysozymes as antibodies?

Lysozymes are non-specific enzymes that attack general bacterial cell wall structures. Antibodies are specific proteins produced by the adaptive immune system that only bind to one specific type of antigen.

Define 'Phagocytosis'.

Phagocytosis is the process by which certain white blood cells (phagocytes) engulf solid particles, such as bacteria, into an internal vesicle to digest and destroy them using enzymes.

What are 'Lysozymes' and where are they found?

Lysozymes are antibacterial enzymes that damage bacterial cell walls. they are found in body secretions such as tears, saliva, and mucus.

Sebum is an oily secretion produced by the sebaceous glands in the skin. It contains chemicals that inhibit the growth of pathogenic bacteria and fungi on the skin's surface.

What is the role of 'Cilia' in the respiratory system?

Cilia are microscopic hair-like projections that waft mucus (containing trapped pathogens) upward toward the throat to be swallowed or expelled, preventing lung infections.

Non-Specific Defences | OCR GCSE Science

1. Definition & Core Concepts

Non-specific immunity refers to the innate protective mechanisms that respond to all pathogens in a uniform manner, regardless of their specific antigens.

These defences are categorized into physical barriers, which mechanically block entry, and chemical barriers, which use biological substances to neutralize or kill microorganisms.

The primary goal of these systems is to prevent the establishment of an infection before the more complex, specific immune response (involving lymphocytes) is required.

2. Physical Barriers to Infection

The Skin: Acts as a continuous, tough outer layer that covers most of the body. It serves as a primary mechanical barrier; if breached, it initiates rapid repair through clotting to restore integrity.
Respiratory Defences: The nasal passages contain hairs and mucus to trap inhaled particles. Further down, the trachea and bronchi are lined with ciliated epithelial cells.
Ciliary Action: These microscopic hair-like structures (cilia) move in a coordinated wave-like motion to 'waft' pathogen-laden mucus away from the lungs toward the throat, where it can be swallowed or expelled.

3. Chemical and Biological Barriers

Stomach Acid: The gastric glands produce hydrochloric acid (HCl), creating a highly acidic environment (low pH) that denatures the proteins of most swallowed pathogens.
Lysozymes: These are specialized enzymes found in tears, saliva, and nasal secretions that chemically break down the cell walls of bacteria, causing them to burst.
Sebum: Produced by sebaceous glands in the skin, this oily substance contains chemicals that inhibit the growth of certain bacteria and fungi.
Natural Flora: Beneficial bacteria residing in the gut and vagina provide biological protection by outcompeting pathogenic species for nutrients and space.

4. Blood Clotting and Wound Repair

When a blood vessel is damaged, platelets (cell fragments) accumulate at the site and release chemicals that trigger a complex cascade of reactions.

The key step involves the conversion of the soluble plasma protein fibrinogen into the insoluble protein fibrin. Fibrin forms a fibrous mesh that traps red blood cells to create a clot.

This clot eventually hardens into a scab, which serves two vital functions: preventing further blood loss and sealing the wound against opportunistic pathogens while new skin grows underneath.

5. The Cellular Response: Phagocytosis

Phagocytes are a type of white blood cell (leukocyte) that identifies, engulfs, and destroys foreign cells through a process called phagocytosis.

They possess a multi-lobed nucleus, which provides the structural flexibility needed for the cell to squeeze through narrow gaps in capillary walls and change shape to surround pathogens.

Once a pathogen is enclosed within a vesicle (phagosome) inside the phagocyte, digestive enzymes are released into the vesicle to break down and kill the microorganism.

Phagocytes are attracted to the site of infection by chemical signals released by damaged tissues or the pathogens themselves, a process known as chemotaxis.

Diagram of a phagocyte extending pseudopodia to engulf a red-colored pathogen, highlighting the multi-lobed nucleus.

6. Key Distinctions

Feature	Non-Specific Defence	Specific Defence
Target	All pathogens (General)	Specific antigens (Targeted)
Speed	Immediate response	Slower (days to weeks)
Memory	No immunological memory	Develops long-term memory
Components	Barriers, Phagocytes, Clotting	Lymphocytes (B and T cells)

It is critical to remember that non-specific defences are the first line (barriers) and second line (phagocytes/inflammation) of the immune system, while specific immunity is the third line.

7. Exam Strategy & Tips

Identify Cell Types: In microscopy questions, look for the multi-lobed nucleus to identify phagocytes. If the nucleus is large and round, it is likely a lymphocyte (specific immunity).
Mechanism Steps: When describing blood clotting, always mention the conversion of soluble fibrinogen to insoluble fibrin. Forgetting the 'soluble/insoluble' distinction often loses marks.
Functional Reasoning: If asked why mucus is moved to the throat, explain that it prevents pathogens from reaching the delicate gas exchange surfaces of the lungs where they could cause systemic infection.
Common Error: Do not confuse 'antibodies' with 'enzymes'. Lysozymes are enzymes (non-specific); antibodies are proteins produced by lymphocytes (specific).

Non-Specific Defences

Summary

1. Definition & Core Concepts

2. Physical Barriers to Infection

3. Chemical and Biological Barriers

4. Blood Clotting and Wound Repair

5. The Cellular Response: Phagocytosis

6. Key Distinctions

7. Exam Strategy & Tips

Non-Specific Defences

Summary

1. Definition & Core Concepts

2. Physical Barriers to Infection

3. Chemical and Biological Barriers

4. Blood Clotting and Wound Repair

5. The Cellular Response: Phagocytosis

6. Key Distinctions

7. Exam Strategy & Tips