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Diseases and Immunity - Body defences

Grade 11A LevelBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Pathogens are microorganisms that cause infectious diseases, including bacteria, viruses, fungi, and protozoa.

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The body has mechanical barriers to prevent entry: the skin (a physical tough outer layer) and hairs in the nose (which trap dust and pathogens).

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Chemical barriers provide protection by killing pathogens: stomach acid (containing HClHCl) kills bacteria in food, and mucus (produced by goblet cells) traps pathogens in the airways.

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Phagocytes are white blood cells that perform phagocytosis. They engulf pathogens into a food vacuole and digest them using enzymes.

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Lymphocytes are white blood cells that produce antibodies. Antibodies have specific shapes that are complementary to the antigens on the surface of a pathogen.

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Antigen-antibody binding leads to the destruction of pathogens through agglutination (clumping) or by marking them for phagocytes.

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Active immunity is long-term defense against a pathogen by the production of antibodies in the body. This can be gained through natural infection or vaccination.

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Vaccination involves introducing weakened or dead pathogens into the body, which stimulates lymphocytes to produce antibodies and, crucially, memory cells.

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Passive immunity is short-term defense acquired by receiving ready-made antibodies (e.g., across the placenta or via breast milk). No memory cells are produced.

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Memory cells remain in the blood and provide a rapid secondary immune response if the same pathogen (AgAg) re-enters the body.

📐Formulae

M=IAM = \frac{I}{A}

Ab+Ag→[AbAg] (Antibody-Antigen Complex)Ab + Ag \rightarrow [AbAg] \text{ (Antibody-Antigen Complex)}

tsecondary response<tprimary responset_{\text{secondary response}} < t_{\text{primary response}}

💡Examples

Problem 1:

An image of a lymphocyte measures 30 mm30\text{ mm} in diameter. If the actual diameter of the lymphocyte is 15 μm15\text{ }\mu m, calculate the magnification used.

Solution:

M=IA=30×103 μm15 μm=2000×M = \frac{I}{A} = \frac{30 \times 10^3\text{ }\mu m}{15\text{ }\mu m} = 2000\times

Explanation:

First, convert the image size from millimeters to micrometers (1 mm=1000 μm1\text{ mm} = 1000\text{ }\mu m). Then divide the image size by the actual size to find the magnification.

Problem 2:

Explain why a child who has had measles is unlikely to catch it again, even if exposed to the virus.

Solution:

The child has developed active immunity. During the first infection, lymphocytes produced memory cells specific to the measles antigens. If exposed again, these memory cells facilitate a much faster and larger production of antibodies (AbAb), destroying the virus before symptoms appear.

Explanation:

This demonstrates the secondary immune response, where the concentration of antibodies rises rapidly compared to the primary response.

Problem 3:

Identify the chemical barrier in the stomach and its effect on pathogens.

Solution:

The stomach contains Hydrochloric acid (HClHCl). The low pHpH (high acidity) denatures the enzymes and proteins of most pathogens ingested with food, effectively killing them.

Explanation:

Chemical barriers use substances with specific chemical properties (like pHpH) to neutralize biological threats before they reach the bloodstream.