Disease & Immunity

Introduction to Disease

A disease is any condition that impairs the normal functioning of the body. Diseases can be broadly categorised as transmissible (infectious) or non-transmissible. A transmissible disease is caused by a microorganism called a pathogen and can be passed from one host to another.

Types of Pathogens

Pathogens are diverse, but the main types you need to know are:

1. Bacteria: These are single-celled prokaryotic organisms. They have a cell wall, cell membrane, cytoplasm, and a circular loop of DNA, but no true nucleus. Bacteria cause disease by reproducing rapidly inside the body and producing toxins.

* Examples: *Vibrio cholerae* causes cholera, and *Mycobacterium tuberculosis* causes tuberculosis (TB).

1. Viruses: Viruses are not considered living cells. They are much smaller than bacteria and consist of a core of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. They are parasites, meaning they can only reproduce inside living host cells, hijacking the cell's machinery to make more viruses. This process often destroys the host cell.

* Examples: HIV (Human Immunodeficiency Virus) causes AIDS, the influenza virus causes flu, and coronaviruses cause diseases like COVID-19.

1. Fungi: Some fungi can be pathogenic to humans, often causing skin infections.

* Example: Athlete's foot is a common fungal infection.

1. Protoctists: These are single-celled eukaryotic organisms. Some are parasitic and cause serious diseases.

* Example: *Plasmodium* is the protoctist that causes malaria. It has a complex life cycle involving two hosts: humans and mosquitoes.

Transmission of Pathogens

For a disease to spread, pathogens must be transmitted from an infected person to an uninfected one. Key methods include:

* Direct Contact: Physical touching, such as shaking hands or contact with infected skin (e.g., athlete's foot).

* Droplet Infection: Inhaling airborne droplets containing pathogens, expelled when an infected person coughs or sneezes (e.g., influenza, TB, COVID-19).

* Contaminated Food and Water: Ingesting food or water containing pathogens, often from faecal contamination. This is a major cause of cholera outbreaks, particularly in areas with poor sanitation, such as after the seasonal floods in parts of Sindh and Punjab.

* Vectors: An animal that transmits a pathogen from one host to another without getting sick itself. The female Anopheles mosquito is a vector for *Plasmodium*, transmitting malaria when it feeds on human blood. Similarly, the *Aedes aegypti* mosquito is a vector for the dengue virus, a major health concern in Pakistani cities.

The Body's Defence Mechanisms

Our bodies have a multi-layered defence system against pathogens.

1. First Line of Defence (Non-specific Barriers):

These barriers aim to prevent pathogens from entering the body.

* Mechanical Barriers: The skin is a tough, physical barrier. Blood clotting seals wounds to prevent entry. Mucus produced in the airways traps pathogens, and cilia (tiny hairs) sweep this mucus up and out.

* Chemical Barriers: Hydrochloric acid in the stomach creates a highly acidic environment (pH 1-2) that kills most ingested pathogens. Tears and saliva contain an enzyme called lysozyme that breaks down bacterial cell walls.

2. Second Line of Defence (The Immune System):

If pathogens get past the first line, the immune system, primarily involving white blood cells (leucocytes), takes over.

* Phagocytosis: This is a non-specific response. Phagocytes are a type of white blood cell that detect and destroy pathogens. The process involves:

1. The phagocyte is attracted to the pathogen.
2. It engulfs the pathogen by changing its shape, enclosing it within a vacuole.
3. Lysosomes inside the phagocyte fuse with the vacuole and release digestive enzymes, which break down and destroy the pathogen.

* Lymphocyte Response (Specific Immunity): This response targets specific pathogens. Lymphocytes are another type of white blood cell.

1. Every pathogen has unique molecules on its surface called antigens.
2. When a lymphocyte with a complementary receptor meets a specific antigen, it becomes activated.
3. It then divides rapidly to produce a large number of cells that produce antibodies.
4. Antibodies are protein molecules with a specific shape that is complementary to the antigen. They bind to the antigens on the pathogen's surface, causing them to clump together (agglutination) or marking them for destruction by phagocytes.

Immunity and Vaccination

Immunity is the body's ability to resist a particular infection.

* Active Immunity: The body produces its own antibodies and memory cells. This provides long-term protection.

* Natural: Gained after being infected with a pathogen and recovering.

* Artificial: Gained through vaccination. A vaccine contains a harmless version of a pathogen or its antigens (e.g., dead, weakened, or just parts of it). This stimulates the immune system to produce antibodies and, crucially, memory cells. If the body is later exposed to the real pathogen, the memory cells allow for a very rapid and large-scale antibody production (the secondary response), destroying the pathogen before symptoms develop.

* Passive Immunity: An individual receives ready-made antibodies. This provides immediate but short-term protection because no memory cells are produced.

* Natural: A baby receives antibodies from its mother across the placenta and through breast milk.

* Artificial: An injection of antibodies, such as anti-venom for a snake bite.

Herd Immunity: When a high percentage of a population is vaccinated, it becomes difficult for a pathogen to spread. This protects vulnerable individuals who cannot be vaccinated (e.g., infants, immunocompromised people).

Controlling Disease with Medicine

Antibiotics are drugs used to treat bacterial infections. They work by disrupting bacterial processes, such as building cell walls (e.g., penicillin) or protein synthesis.

* Common Misconception/Exam Trap: Antibiotics are ineffective against viruses. This is because viruses live inside host cells and do not have their own cell walls or metabolic machinery for antibiotics to target.

* Antibiotic Resistance: Overuse and misuse of antibiotics can lead to antibiotic resistance. Within a bacterial population, some individuals may have a natural resistance due to mutation. When antibiotics are used, the non-resistant bacteria are killed, but the resistant ones survive and reproduce, passing on their resistance. This leads to the evolution of 'superbugs' that are very difficult to treat.