Disease

A disease is a condition that impairs the normal functioning of an organism. Many diseases are transmissible, meaning they are caused by a pathogen that can be passed from one individual (the host) to another. A pathogen is a microorganism that causes disease. There are four main types of pathogens you need to know:

1. Bacteria: Single-celled organisms that can reproduce rapidly. They cause disease by damaging cells or releasing harmful toxins (e.g., Salmonella, which causes food poisoning).
2. Viruses: Much smaller than bacteria, viruses are not truly alive. They invade host cells and use the cell's machinery to replicate, which destroys the host cell (e.g., Influenza virus, HIV).
3. Fungi: Some fungi are parasitic and cause disease by growing on and into tissues (e.g., Athlete's foot fungus).
4. Protoctista: A group of single-celled organisms, some of which are parasites that live inside other organisms (e.g., Plasmodium, which causes malaria).

Pathogens can be transmitted in several ways:

• Direct Contact: Touching an infected person (e.g., skin to skin).
• Indirectly: Through a contaminated medium, such as:
• Air: Inhaling droplets from coughs or sneezes (e.g., common cold, influenza).
• Water: Drinking contaminated water (e.g., cholera).
• Food: Eating contaminated food (e.g., Salmonella).
• Vectors: Being bitten by an organism that carries the pathogen (e.g., mosquitoes carrying malaria).

Your body has a set of barriers designed to stop pathogens from getting inside in the first place. This is the first line of defence and it is non-specific, meaning it protects against all types of pathogens. These defences can be mechanical (physical barriers) or chemical (substances that kill pathogens).

Mechanical Barriers:

• Skin: A tough, waterproof outer layer that most pathogens cannot penetrate as long as it is intact. It also produces antimicrobial secretions.
• Hairs in the nose: These trap larger particles and pathogens as you breathe in.
• Cilia and Mucus: The trachea and bronchi are lined with tiny hairs called cilia and cells that produce sticky mucus. The mucus traps pathogens and dust. The cilia then beat in a coordinated way to sweep the mucus up to the throat, where it is swallowed. This is often called the 'mucus escalator'.

Chemical Barriers:

• Stomach Acid: The stomach produces hydrochloric acid, which is strong enough to kill most pathogens that are swallowed in food or mucus.
• Tears and Saliva: These contain an enzyme called lysozyme, which breaks down the cell walls of bacteria, killing them.

If pathogens get past the first line of defence, the second line of defence is activated. This involves specialised white blood cells that actively seek and destroy invaders. There are two main types of white blood cells you need to know about: phagocytes and lymphocytes.

Phagocytes: These are 'eating cells'. They are attracted to pathogens in the blood and tissues. A phagocyte will flow around a pathogen, enclosing it within a vacuole. The phagocyte then releases digestive enzymes into the vacuole to break down and destroy the pathogen. This entire process is called phagocytosis.

Lymphocytes: These are 'intelligence cells' that coordinate the immune response. They have two key functions:

1. Producing Antibodies: Lymphocytes recognise specific markers (antigens) on the surface of pathogens. They then produce proteins called antibodies that are a perfect complementary shape to these antigens. These antibodies lock onto the pathogens, which helps to neutralise them or mark them for destruction by phagocytes.
2. Producing Antitoxins: Some bacteria produce harmful toxins. Lymphocytes can produce antitoxins, which are specific molecules that bind to and neutralise these toxins, rendering them harmless.

The key to the specific immune response lies in the interaction between antigens and antibodies. Every pathogen has unique molecules on its surface, usually proteins, called antigens. Your immune system, specifically the lymphocytes, can recognise these antigens as 'foreign'.

When a lymphocyte with a receptor that matches a specific antigen encounters that pathogen, it is activated. This lymphocyte then divides rapidly to produce a large number of identical cells. Most of these cells become plasma cells, which are antibody factories. They produce huge quantities of antibodies that are a specific, complementary shape to the antigen.

Antibodies do not kill pathogens directly. Instead, they circulate in the blood and help clear the infection in several ways:

1. Agglutination: Antibodies can cause pathogens to clump together. This makes them too large to move around easily and makes it easier for phagocytes to find and engulf them.
2. Marking for Destruction: Antibodies act like labels, marking the pathogens for destruction by phagocytes.
3. Neutralisation: Antibodies can bind to toxins produced by bacteria, preventing them from harming cells. These are called antitoxins.

Because each antibody is specific to one type of antigen, the antibodies that protect you from chickenpox will not work against the measles virus. This is called specificity.

Active immunity is when your own immune system produces antibodies and memory cells in response to an antigen. This provides long-term protection. It can be acquired in two ways:

1. Natural Active Immunity: Gained after being infected with a pathogen and recovering.
2. Artificial Active Immunity: Gained through vaccination.

A vaccine contains a safe version of a pathogen. This could be a dead pathogen, a weakened (attenuated) pathogen, or just some of its antigens. When injected, the vaccine triggers an immune response without causing the disease.

The Immune Response to Vaccination:

• Primary Response: The first time your immune system encounters the antigens (from the vaccine), it takes time for lymphocytes to identify the antigen and produce the correct antibodies. During this time, some lymphocytes become memory cells. Antibody levels rise slowly and then fall.
• Secondary Response: If you are later infected with the real pathogen, the memory cells recognise the antigens immediately. They allow for a much faster and stronger immune response. Antibodies are produced so quickly and in such large numbers that the pathogen is destroyed before you can even feel ill. This is what gives you long-term immunity.

When a large percentage of a population is vaccinated, it creates herd immunity, which protects vulnerable individuals who cannot be vaccinated.

Passive immunity is a type of short-term immunity gained by receiving ready-made antibodies from an external source. Unlike active immunity, your body does not produce the antibodies itself, and critically, no memory cells are made. This means the protection is immediate but temporary.

There are two main ways to acquire passive immunity:

1. Natural Passive Immunity: A baby receives antibodies from its mother. Before birth, antibodies pass from the mother's blood to the baby's blood across the placenta. After birth, breast milk (especially the first milk, colostrum) is rich in antibodies. This protects the baby from diseases to which the mother is immune, during the first few months of life while the baby's own immune system is still developing.
2. Artificial Passive Immunity: This is given as an emergency treatment after a potential exposure to a dangerous pathogen. For example, if you are bitten by a venomous snake or a rabid animal, you can be injected with antibodies (called antiserum or antitoxin) against the venom or virus. This provides immediate neutralisation of the threat.

Because the individual's own immune system has not been activated to produce memory cells, once the 'borrowed' antibodies are broken down and cleared from the body, the immunity is lost.

Preventing disease is just as important as curing it. The spread of transmissible diseases can be limited by breaking the chain of transmission. This is achieved through public health measures focusing on hygiene, sanitation, and reducing contact with pathogens.

1. Hygienic Practices:

• Personal Hygiene: Simple actions like washing hands with soap, especially after using the toilet and before handling food, removes pathogens.
• Food Hygiene: Preventing food-borne diseases involves cooking food thoroughly to kill bacteria, storing it at correct temperatures (e.g., refrigerating) to slow bacterial growth, and avoiding cross-contamination between raw and cooked foods.

2. Improving Sanitation:

• Waste Disposal: Proper disposal of rubbish prevents it from attracting pests like rats and flies, which can be vectors for disease.
• Sewage Treatment: Treating sewage effectively is crucial for preventing water-borne diseases. Raw sewage contains pathogens from human faeces. If it enters the water supply, it can lead to outbreaks of diseases like cholera and typhoid. Sewage treatment plants remove solids and kill pathogens before the water is returned to the environment.

3. Providing Clean Water:

• Water is purified before being supplied to homes. This involves filtration to remove particles and sterilisation (often using chlorine) to kill any remaining pathogens.

These measures are vital for preventing large-scale epidemics and maintaining the health of a population.

Usually, the immune system is excellent at distinguishing between 'self' (your own body cells) and 'non-self' (pathogens and foreign material). However, sometimes it makes a mistake. An autoimmune disease is a condition where the immune system mistakenly attacks and destroys the body's own healthy tissues.

Type 1 Diabetes is a key example of an autoimmune disease. In this condition, the immune system specifically targets and destroys the insulin-producing cells in the pancreas. These cells are called the islets of Langerhans.

• Cause: The body's own lymphocytes identify the insulin-producing cells as foreign and mount an immune attack against them.
• Effect: As these cells are destroyed, the pancreas can no longer produce insulin.
• Consequence: Insulin is a hormone vital for controlling blood glucose levels. It allows glucose to move from the blood into cells to be used for respiration. Without insulin, blood glucose levels become dangerously high, while cells are starved of energy.

It is important not to confuse this with Type 2 diabetes, which is primarily caused by the body's cells becoming resistant to the effects of insulin (often linked to lifestyle factors) rather than an autoimmune attack.