Body Defence Against Pathogens

First line: physical and chemical barriers

• Unbroken skin is a physical barrier that keeps most pathogens from entering the body, and its slightly acidic secretions inhibit bacterial growth.
• Mucus in the airways traps inhaled pathogens and dust, and cilia lining the airways sweep the mucus, together with trapped pathogens, up and out.
• Tears and saliva contain the enzyme lysozyme, which breaks down the cell walls of many bacteria.
• Hydrochloric acid in the stomach kills most pathogens swallowed with food or water.
• Blood clotting quickly seals a wound, closing off an entry point for pathogens.

Second line: phagocytes and inflammation

If pathogens get past the barriers of the first line, phagocytes, a type of white blood cell, engulf and digest them in a non-specific process called phagocytosis, meaning they attack any pathogen the same way. Damaged tissue also triggers inflammation: blood vessels near the site widen and become more permeable, increasing blood flow so more phagocytes can reach the area.

This causes the redness, heat, swelling and pain typical of an infected wound.

Third line: lymphocytes

If pathogens are not fully cleared by the first two lines, lymphocytes provide a slower but highly specific response. B-lymphocytes recognise a particular antigen on the pathogen's surface and produce matching antibodies against it, while T-lymphocytes can destroy body cells that have already been infected.

Unlike the first two lines, this response targets one specific pathogen and can leave behind memory cells that respond faster if the same pathogen is met again.

How it is examined

Active and passive immunity

The specific response of the third line leaves a person immune to a pathogen, and this immunity is described as active or passive. Active immunity develops when the body makes its own antibodies after meeting an antigen, either through infection or through a vaccine; it is slow to build but long-lasting, because memory cells remain ready to respond faster if the pathogen returns.

Passive immunity is gained when ready-made antibodies enter the body without it making them, for example antibodies passed from a mother to her baby across the placenta and in breast milk, or antibodies given by injection. It gives immediate protection but is short-lived, because no memory cells are formed and the borrowed antibodies are gradually broken down.

Worked exam-style question

Question. A person cuts their hand on a rusty nail and the wound becomes infected with bacteria. (a) Name the first line of defence that was broken.

(b) Describe how phagocytes deal with the bacteria that enter. (c) Explain how the third line of defence gives longer-term protection against the same bacteria.

(d) The person is later given a vaccine against these bacteria. Explain how the vaccine produces immunity.

Model answer. (a) The skin, a physical barrier of the first line of defence. (b) Phagocytes engulf and digest the bacteria in a non-specific process called phagocytosis, attacking any pathogen the same way.

(c) B-lymphocytes recognise a specific antigen on the bacteria and produce matching antibodies against it; some become memory cells that respond faster and produce antibodies more quickly if the same bacteria return. (d) A vaccine contains a weakened or dead form of the pathogen (or its antigen), which triggers the body to make antibodies and memory cells without causing the disease, giving active immunity.

Practice question

Try this. A newborn baby receives antibodies from its mother through the placenta and later in breast milk. (a) State whether this is active or passive immunity.

(b) Explain your answer. (c) State one difference between this type of immunity and the immunity produced by a vaccine.

Exam tip

Key terms

• Pathogen, a microorganism that causes disease.
• Immunity, the ability of the body to resist a particular pathogen.
• Antigen, a molecule on a pathogen that triggers an immune response.
• Antibody, a protein made by lymphocytes that binds to a specific antigen.
• Vaccine, a preparation of a weakened or dead pathogen that produces active immunity.

Source:SRC-DSKP-EN

Frequently asked questions

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