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How does the body defend itself against pathogens before the adaptive immune response is mounted?

Describe the innate (non-specific) defences, including barriers, phagocytosis and the inflammatory response

A focused answer to the H2 Biology Infectious Disease and Immunity outcome on innate immunity. The physical and chemical barriers, phagocytosis by neutrophils and macrophages, the inflammatory response, and why innate immunity is fast but non-specific.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the innate (non-specific) defences: the physical and chemical barriers that keep pathogens out, phagocytosis that engulfs and destroys those that get in, and the inflammatory response. You should understand that innate immunity is fast and general, in contrast to the slower, specific adaptive response that follows.

The answer

Barriers: the first line of defence

The body's first defences are non-specific barriers that stop pathogens entering:

  • Physical barriers such as the skin and the mucus lining of the airways (which traps pathogens, swept away by cilia).
  • Chemical barriers such as stomach acid (which kills swallowed pathogens) and enzymes in tears and saliva.

Phagocytosis: the second line of defence

If pathogens enter, phagocytes (neutrophils and macrophages) destroy them by phagocytosis:

  1. The phagocyte is attracted to the pathogen by chemicals (chemotaxis) and recognises and binds it.
  2. It engulfs the pathogen by endocytosis, forming a vesicle (a phagosome).
  3. A lysosome fuses with the phagosome and releases hydrolytic enzymes.
  4. The enzymes digest and destroy the pathogen.

The inflammatory response

At a site of infection, cells release signalling molecules such as histamine. These cause local blood vessels to dilate and become more permeable, increasing blood flow (redness, heat) and letting plasma and phagocytes enter the tissue (swelling). This delivers more phagocytes to destroy pathogens.

Fast but non-specific

Innate defences act quickly and the same way against any pathogen. They do not target specific pathogens and do not improve with repeated exposure, which is the job of the adaptive response.

Examples in context

Example 1. Mucus and cilia in the airways. The respiratory tract is lined with mucus that traps inhaled pathogens, and cilia sweep the mucus up to be swallowed or expelled. This barrier explains why damage to cilia (for example by smoking) makes lung infections more common.

Example 2. Antigen presentation. After engulfing a pathogen, a macrophage presents the pathogen's antigens on its surface, activating helper T lymphocytes. This bridges the innate and adaptive responses and is why phagocytosis is central to the immune system as a whole.

Try this

Q1. State one physical and one chemical barrier that form the first line of defence. [2 marks]

  • Cue. Physical: skin or mucus lining the airways. Chemical: stomach acid or enzymes in tears and saliva.

Q2. Name the organelle that fuses with the phagosome to destroy an engulfed pathogen. [1 mark]

  • Cue. A lysosome (releasing hydrolytic enzymes).

Q3. Explain why the innate immune response is described as non-specific. [1 mark]

  • Cue. It responds in the same general way to any pathogen, without targeting a particular one or improving with repeated exposure.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksDescribe the process of phagocytosis and explain how it contributes to the destruction of pathogens.
Show worked answer →

Examiners want the ordered steps of phagocytosis and the role of enzymes.

A phagocyte, such as a neutrophil or macrophage, is attracted to a pathogen by chemicals released at the site of infection (chemotaxis). The phagocyte recognises and binds to the pathogen, often where the pathogen has been marked by antibodies or other proteins.

The phagocyte engulfs the pathogen by endocytosis, surrounding it with its cell surface membrane and enclosing it in a vesicle called a phagosome inside the cell.

A lysosome fuses with the phagosome and releases hydrolytic (digestive) enzymes into it. These enzymes break down and destroy the pathogen. The harmless products are then absorbed or removed.

Markers reward attraction to the pathogen, recognition and binding, engulfing by endocytosis to form a phagosome, fusion with a lysosome, and digestion by hydrolytic enzymes.

Original4 marksExplain the changes that occur during the inflammatory response and how they help the body deal with infection.
Show worked answer →

The answer should describe the changes and their protective benefit.

When tissue is infected or damaged, cells release signalling molecules such as histamine. These cause the local blood vessels to dilate (widen) and become more permeable (leaky).

Dilation increases blood flow to the area, causing redness and heat, and the increased permeability allows plasma and white blood cells, especially phagocytes, to leave the blood and enter the tissue, causing swelling. The increased delivery of phagocytes means more pathogens are engulfed and destroyed, and the raised temperature can inhibit pathogen growth.

Markers reward the release of histamine, vasodilation and increased permeability, the resulting delivery of phagocytes and plasma to the site, and the benefit of more effective pathogen destruction.

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