11.1 Body Defence
The human body is constantly exposed to various microorganisms in the environment. Pathogens such as bacteria, viruses, and parasites can cause diseases if they successfully enter and infect body cells. To protect against these threats, the human body is equipped with a defence system organised into three lines of defence: the first line, the second line, and the third line of defence. Understanding how these defence mechanisms function helps us appreciate how immunity is established and maintained.
Learning Objectives
- Define pathogen, antigen, antibody, immune response, and immunity.
- Describe the components and mechanisms of the first line of defence (skin, mucous membrane, lysozyme, hydrochloric acid, and blood clotting mechanism).
- Explain the components and mechanisms of the second line of defence (fever, phagocytosis, and inflammation).
- Describe the stages of phagocytosis carried out by phagocytes.
- Explain the response of the third line of defence involving T lymphocytes, B lymphocytes, and antibody production.
Summary of the Three Lines of Human Body Defence
A structural overview of non-specific versus specific defences, categorising the barriers and responses in each line.
First Line
Skin, mucous membrane, lysozyme, stomach acid, blood clotting.
Second Line
Fever, phagocytosis, inflammation.
Third Line
T and B lymphocytes, antibody-antigen binding.
Short Explanation
Fundamental Definitions
Pathogen is a microorganism that causes diseases. Antigen is a foreign particle that enters the body and subsequently stimulates an immune response. Antibody is a protein found on the lymphocyte surface or released by lymphocytes into the blood plasma to destroy antigens. Immune response is the interaction between antibodies and antigens that causes antigens to be destroyed. Immunity is the body's ability to fight infections caused by pathogens or other foreign objects through specific attacks on the pathogens.
First Line of Defence
Non-specific physical and chemical barriers prevent pathogens from entering the body. The skin is tough and sheds dead cell layers, secretes sebum containing oil and acid, and sweat contains lysozyme. Mucous membranes secrete sticky mucus with lysozyme. Hydrochloric acid in the stomach destroys bacteria in food and drinks. Blood clotting prevents bacteria from entering through open wounds.
Second Line of Defence
Activated when pathogens penetrate the first line. Fever increases phagocytic activity. Phagocytosis traps and digests microorganisms by phagocytes such as neutrophils and monocytes. Inflammation is an immediate response that destroys and neutralises harmful actions of microorganisms and toxins at early stages of infection.
Third Line of Defence
Activated if pathogens overcome the second line. It is the specific immune response of lymphocytes. T lymphocytes attack infected cells and stimulate B lymphocytes to produce memory cells. B lymphocytes produce antibodies and memory cells. Memory cells are immediately stimulated to produce antibodies rapidly if the same pathogen attacks again.
Stages of Phagocytosis
1. Phagocyte extends pseudopodium towards the bacterium and envelops it. 2. Ingestion forms a phagosome. 3. Phagosome combines with a lysosome that secretes lysozyme, forming a phagolysosome. 4. Bacterium is destroyed and decomposed by lysozyme. 5. Phagocyte expels the remains of the digested microorganism out of the cell.
Try to Answer First
Answer in your mind, then press "Check Answer".
What is the meaning of immunity?
Name two characteristics of the skin that allow it to act as an effective first line of defence.
How does a phagocyte destroy pathogens that have penetrated the first line of defence?
Common Mistakes
- Confusing the terms antigen and antibody: students often write that antigens destroy antibodies, whereas antibodies are proteins produced by lymphocytes to destroy foreign antigens.
- Stating that the first and second lines of defence are specific: both the first and second lines are non-specific, while only the third line of defence is specific.
- Forgetting the role of lysosomes in phagocytosis: students sometimes state that the phagocyte directly decomposes bacteria without mentioning the lysosome releasing lysozyme into the phagosome.
Concept Misunderstandings
Fever is harmful and should always be stopped immediately because it is a disease.
Fever is an active second line of defence mechanism that increases phagocytic activity to help fight infecting microorganisms.
Inflammation only causes damage and pain without any protective function.
Inflammation expands blood capillaries and increases their permeability so that phagocytes and clotting factors can rapidly reach the infected site to eliminate pathogens.
Summary
- First Line (Non-specific): Physical barriers (skin, blood clotting) and chemical barriers (lysozyme in tears/saliva/mucus/sweat, sebum, stomach hydrochloric acid) prevent pathogen entry.
- Second Line (Non-specific): Fever, phagocytosis by neutrophils and monocytes, and inflammation triggered by histamine destroy pathogens that have entered the body.
- Third Line (Specific): T lymphocytes attack infected cells and stimulate B lymphocytes to produce specific antibodies and memory cells against specific antigens.
Short Activity
Objective: test your understanding of body defence, phagocytosis, and specific immune responses.
A. Objective Quiz
Which of the following white blood cells are phagocytes involved in the second line of defence?
What substance is released by damaged tissues to stimulate an immediate inflammatory response?
B. Fill in the Blanks
An enzyme found in tears, saliva, nasal mucus, and sweat that breaks down bacterial cell walls is called .
During phagocytosis, a phagosome combines with a lysosome to form a where bacteria are destroyed.
C. Matching / Drag and Drop
Drag each defence component to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Defence Component to Its Description
Write the correct defence component next to each description.
| Description | Defence Component |
|---|---|
| 1. Tough physical barrier with continuous shedding of dead cells and secretion of sebum | |
| 2. Chemical defence in the stomach that destroys bacteria in food and drinks | |
| 3. Attacks infected cells and stimulates B lymphocytes to produce memory cells | |
| 4. Chemical released by damaged tissues that causes capillaries to expand and increase permeability |
Answer choices: Skin • Hydrochloric acid • T lymphocyte • Histamine
