Immunity and Defence against Disease

What you'll learn

This revision guide covers how the human body defends itself against disease-causing organisms. You will learn about the body's natural barriers, the immune response, and how vaccination provides protection. This topic is essential for the CSEC Integrated Science examination and connects to public health issues affecting Caribbean communities.

Key terms and definitions

Pathogen — a disease-causing microorganism such as bacteria, viruses, fungi or protists

Antibody — a protein produced by white blood cells that binds to specific antigens on pathogens to destroy them

Antigen — a foreign protein or marker on the surface of a pathogen that triggers an immune response

Phagocytosis — the process by which white blood cells engulf and digest pathogens

Immunity — the body's ability to resist infection by a specific pathogen through the presence of antibodies and memory cells

Vaccination — the introduction of weakened or dead pathogens into the body to stimulate antibody production without causing disease

Lymphocyte — a type of white blood cell that produces antibodies specific to particular antigens

Active immunity — long-lasting immunity developed when the body produces its own antibodies in response to an antigen

Core concepts

The body's first line of defence

The human body has several physical and chemical barriers that prevent pathogens from entering:

Skin acts as a physical barrier. The outer layer of dead cells is difficult for pathogens to penetrate. Sebaceous glands in the skin produce sebum, an oily substance that contains chemicals which kill bacteria and fungi.

Mucus is produced by cells lining the respiratory tract, digestive system and reproductive organs. This sticky substance traps pathogens and particles before they can enter the body. In the respiratory system, cilia (tiny hair-like structures) sweep the mucus upward toward the throat where it is swallowed.

Stomach acid creates a highly acidic environment (pH 1-2) that kills most pathogens that enter through the mouth with food or water. This is particularly important in Caribbean countries where foodborne illnesses can be transmitted through contaminated street food or improperly stored provisions.

Tears and saliva contain the enzyme lysozyme, which breaks down bacterial cell walls. Tears also wash away pathogens from the surface of the eyes.

Blood clotting seals wounds quickly to prevent pathogen entry. When skin is cut, platelets and clotting proteins form a scab that acts as a temporary barrier while new skin cells grow underneath.

The second line of defence: non-specific immune response

If pathogens pass the first line of defence, the body activates non-specific responses that attack all foreign invaders:

White blood cells (phagocytes) patrol the bloodstream and tissues. When they encounter a pathogen, they engulf it through phagocytosis:

1. The phagocyte detects chemicals released by the pathogen
2. The cell membrane extends around the pathogen
3. The pathogen is enclosed in a vacuole inside the phagocyte
4. Enzymes are released into the vacuole
5. The pathogen is digested and broken down

Inflammation occurs at the site of infection or injury. Blood vessels dilate (widen) and become more permeable, allowing more white blood cells and antibodies to reach the affected area. This causes redness, heat, swelling and pain. Inflammation is why mosquito bites common throughout the Caribbean become red and swollen — the body is responding to proteins injected by the mosquito.

Fever raises body temperature, which can slow pathogen reproduction and speed up the body's metabolic processes. Many enzymes in the immune system work more efficiently at higher temperatures.

The third line of defence: specific immune response

The lymphocytes provide a targeted defence against specific pathogens. This response takes longer to activate but is highly effective:

B-lymphocytes produce antibodies. Each B-lymphocyte produces antibodies with a unique shape that matches a specific antigen. When the correct B-lymphocyte encounters its matching antigen:

1. The B-lymphocyte recognizes the antigen
2. The cell divides rapidly to produce many identical copies (clones)
3. These clones produce large quantities of the specific antibody
4. Antibodies bind to antigens on the pathogen surface
5. This marks pathogens for destruction by phagocytes or causes them to clump together

T-lymphocytes coordinate the immune response and directly attack infected cells. Some T-lymphocytes help activate B-lymphocytes, while others destroy body cells that have been infected by viruses.

Memory cells remain in the bloodstream after an infection is cleared. These are long-lived B and T-lymphocytes that "remember" the antigen. If the same pathogen enters the body again, memory cells trigger a faster, stronger immune response, often preventing symptoms from developing. This is the basis of immunity.

Active and passive immunity

Active immunity develops when the body produces its own antibodies. This can occur through:

• Natural active immunity: infection by a pathogen stimulates antibody production. For example, after recovering from dengue fever (common in Trinidad, Jamaica, Barbados and other Caribbean territories), a person develops immunity to that specific dengue virus strain.

• Artificial active immunity: vaccination introduces dead, weakened or parts of pathogens. The immune system responds by producing antibodies and memory cells without the person suffering from the disease. This provides long-lasting protection.

Active immunity takes time to develop (1-2 weeks) but can last for years or even a lifetime.

Passive immunity occurs when antibodies are received from another source:

• Natural passive immunity: antibodies pass from mother to baby through the placenta during pregnancy and through breast milk after birth. This protects infants during their first months before their immune system fully develops.

• Artificial passive immunity: antibodies are injected directly, providing immediate but temporary protection. This is used for emergency treatment of snake bites (important in regions like Guyana where venomous snakes are present) or rabies exposure.

Passive immunity provides immediate protection but lasts only a few weeks or months because the body does not produce memory cells.

Vaccination programmes

Vaccination has eliminated or reduced many diseases in the Caribbean. The Caribbean Public Health Agency (CARPHA) coordinates vaccination programmes across member states.

How vaccines work:

1. Dead, weakened or fragments of a pathogen are injected
2. Antigens on the vaccine trigger an immune response
3. B-lymphocytes produce antibodies
4. Memory cells are formed
5. If the real pathogen enters later, memory cells respond rapidly

Common vaccines in Caribbean programmes include:

• MMR (measles, mumps, rubella)
• BCG (tuberculosis)
• Polio
• Hepatitis B
• HPV (human papillomavirus)
• Influenza (for high-risk groups)

Herd immunity occurs when a large percentage of a population is vaccinated. This protects unvaccinated individuals (babies, people with weakened immune systems) because the disease cannot spread easily. In small Caribbean island nations, maintaining herd immunity is particularly important due to close-knit communities and high population density in urban areas.

Challenges to vaccination in the Caribbean:

• Vaccine storage requires reliable refrigeration (cold chain), which can be disrupted by hurricanes or infrastructure problems
• Misinformation about vaccine safety spreads through social media
• Access to remote communities on larger islands or between islands
• Cost and supply of vaccines to smaller territories

Antibiotics and their limitations

Antibiotics are medicines that kill bacteria or prevent their reproduction. They are effective against bacterial infections like strep throat, urinary tract infections and some types of pneumonia.

Antibiotics work by:

• Damaging bacterial cell walls
• Interfering with bacterial protein synthesis
• Disrupting bacterial DNA replication

Critical limitations:

Antibiotics are completely ineffective against viruses. Viruses live inside host cells and use the cell's machinery to reproduce. Antibiotics cannot distinguish between host cells and viruses. Common viral infections like colds, flu, dengue fever and chikungunya (prevalent in the Caribbean) cannot be treated with antibiotics.

Antibiotic resistance develops when bacteria evolve to survive antibiotic treatment. This occurs when:

• Patients do not complete their full course of antibiotics
• Antibiotics are used unnecessarily (e.g., for viral infections)
• Antibiotics are used in agriculture and animal farming

Resistant bacteria survive and reproduce, passing resistance genes to their offspring. In Caribbean countries where antibiotics can sometimes be purchased without prescription, antibiotic resistance is a growing concern. Methicillin-resistant Staphylococcus aureus (MRSA) has been detected in Caribbean hospitals.

Preventing antibiotic resistance:

• Only use antibiotics when prescribed by a doctor
• Complete the full course even if symptoms improve
• Never share antibiotics or use leftover medications
• Doctors should only prescribe antibiotics for bacterial infections

Worked examples

Example 1: Immune response to infection

Question: A student cuts their finger while preparing vegetables in Food and Nutrition class. The wound becomes infected with bacteria. Describe how the student's body responds to this infection. (6 marks)

Model answer:

First line of defence: Blood clotting occurs at the wound site, forming a scab that prevents more bacteria from entering (1 mark).

Non-specific response: Phagocytes move to the infected area and engulf bacteria through phagocytosis (1 mark). The area becomes inflamed — red, warm and swollen as blood vessels dilate and more white blood cells arrive (1 mark).

Specific response: B-lymphocytes recognize antigens on the bacterial surface (1 mark). They divide to produce clones that release antibodies specific to these bacteria (1 mark). The antibodies bind to the bacteria, marking them for destruction by phagocytes. Memory cells remain in the blood to provide immunity against future infection by the same bacteria (1 mark).

Example 2: Comparing active and passive immunity

Question: A baby born in Barbados receives antibodies from her mother through breast milk. Later, she receives vaccinations as part of the national immunization programme.

(a) Name the type of immunity provided by breast milk. (1 mark)

(b) Explain why this immunity is only temporary. (2 marks)

(c) Explain how vaccination provides longer-lasting protection than breast milk. (3 marks)

Model answer:

(a) Natural passive immunity (1 mark)

(b) The baby receives antibodies made by the mother's immune system (1 mark). The baby's body does not produce its own antibodies or memory cells, so protection only lasts while the received antibodies remain active in her bloodstream, usually a few months (1 mark).

(c) Vaccination contains dead or weakened pathogens that stimulate the baby's own immune system (1 mark). The baby's B-lymphocytes produce antibodies specific to the vaccine antigens (1 mark). Memory cells are formed that remain in the body for years or a lifetime, providing long-term immunity (1 mark).

Example 3: Antibiotic misuse

Question: A farmer in St. Lucia has a viral infection causing a sore throat and fever. He takes leftover antibiotics from a previous illness but stops after three days when he feels better.

(a) Explain why the antibiotics will not cure his viral infection. (2 marks)

(b) Explain how the farmer's actions could contribute to antibiotic resistance. (3 marks)

Model answer:

(a) Antibiotics only work against bacteria by damaging bacterial structures or processes (1 mark). Viruses live inside host cells and do not have the same structures as bacteria, so antibiotics cannot affect them (1 mark).

(b) If the farmer had a bacterial infection remaining from before, stopping treatment early would kill only the weakest bacteria (1 mark). The strongest, most resistant bacteria would survive and multiply (1 mark). These resistant bacteria could spread to others or cause a new infection that antibiotics cannot treat (1 mark).

Common mistakes and how to avoid them

• Confusing antibodies and antigens: Remember that antigens are markers on pathogens that trigger the response, while antibodies are proteins produced by the body that bind to antigens. Think: antigens are the enemy's uniform, antibodies are the weapons that recognize that uniform.

• Saying antibiotics kill viruses: This is incorrect and frequently tested. Antibiotics only affect bacteria. Viral infections must be fought by the immune system alone or with antiviral medications (which work differently).

• Not distinguishing between active and passive immunity: Active immunity involves the body producing its own antibodies and memory cells (long-lasting). Passive immunity involves receiving antibodies from elsewhere (temporary, no memory cells).

• Mixing up the roles of B and T-lymphocytes: B-lymphocytes produce antibodies. T-lymphocytes help coordinate the response and kill infected cells. You may only need to know about B-lymphocytes for CSEC level — check your syllabus.

• Forgetting that vaccination uses weakened or dead pathogens: Students sometimes write that vaccines inject antibodies — this is passive immunity, not vaccination. Vaccines contain pathogen material that stimulates your own antibody production.

• Not explaining why memory cells provide immunity: State clearly that memory cells allow faster production of antibodies if the same pathogen returns, often preventing symptoms altogether.

Exam technique for "Immunity and Defence against Disease"

• Command word awareness: "Describe" requires you to state what happens in sequence. "Explain" requires you to say why or how something happens with reasoning. For a 4-mark "explain" question, give at least four separate points linked by cause and effect.

• Use biological terms precisely: Write "phagocytosis" not "eating," "antibodies" not "proteins that fight disease," "antigens" not "germs." Proper terminology earns marks and shows understanding.

• Sequence matters for immune response: Structure answers logically: first barrier → non-specific response → specific response → memory cells. This organization helps you remember all components and makes your answer clear for examiners.

• Caribbean context questions: Be prepared to apply knowledge to regional scenarios like dengue outbreaks, vaccination campaigns, or traditional remedies. Use your scientific knowledge to explain why these work or don't work, always referring back to immune system mechanisms.

Quick revision summary

The body defends against pathogens through three levels: physical/chemical barriers (skin, mucus, stomach acid), non-specific responses (phagocytosis, inflammation), and specific immune responses (lymphocytes producing antibodies against antigens). Active immunity develops when the body produces its own antibodies, either through infection or vaccination, creating memory cells for long-term protection. Passive immunity provides temporary protection through received antibodies. Antibiotics kill bacteria but not viruses; misuse leads to antibiotic resistance. Vaccination programmes create herd immunity, protecting Caribbean communities from preventable diseases.