B13 Notes - Diseases and Immunity

Pathogens and Disease

  • Pathogen Definition: A disease-causing organism.

  • Transmissible Diseases: These are diseases that pathogens can transfer from one host to another.

  • Modes of Transmission:

    • Direct contact (e.g., through blood and body fluids).

    • Indirect contact (e.g., surfaces, food, animals, air).

  • Importance of Controls: Control of disease spread involves:

    • Clean water supply

    • Hygienic food preparation

    • Personal hygiene

    • Effective waste disposal

    • Sewage treatment

  • Types of Pathogens:

    • Bacteria:

      • A type of single-celled microorganism that can exist either as free-living organisms or as parasites that require a host.

      • Example: Escherichia coli

      • Diseases caused: Strep throat, tuberculosis, food poisoning.

    • Viruses:

      • Infectious agents that are not considered living organisms. They require a host cell to reproduce and sustain themselves.

      • Example: Herpes simplex

      • Diseases caused: Common cold, flu, AIDS.

    • Fungi:

      • A diverse group of organisms that can be unicellular or multicellular, known for their role in decomposition and their ability to cause infections.

      • Example: Death cap mushroom

      • Diseases caused: Ringworm, athlete's foot.

    • Protozoa:

      • A group of single-celled eukaryotic organisms, often free-living, but can also be parasitic in nature.

      • Example: Giardia lamblia

      • Diseases caused: Malaria, giardiasis.

Body’s Defences

  • Defense Mechanisms: The body employs various defense mechanisms to combat infections, including physical barriers (such as skin and mucous membranes), innate immunity (like phagocytosis and inflammation), and adaptive immunity (which involves the production of antibodies and memory cells).

    Three Lines of Defence:

    1. First Line of Defence: Physical and Chemical Barriers

      • Skin: Acts as a protective barrier that prevents pathogens from entering the body. It has multiple layers which make it difficult for pathogens to penetrate.

      • Nose Hairs and Mucus: Nose hairs trap larger particles, while mucus in the respiratory tract captures pathogens and particles to be expelled or swallowed.

      • Stomach Acid: The highly acidic environment of the stomach (pH 1.5 to 3.5) destroys pathogens that are ingested with food and drink.

    2. Second Line of Defence: Immune Response

      • Phagocytosis: This process involves white blood cells (leukocytes), particularly phagocytes, that engulf and digest pathogens. Upon encountering pathogens, these cells identify, engulf, and destroy them through enzymatic action.

    3. Third Line of Defence: Adaptive Immune Response

      • Antibody Production: This involves the activation of lymphocytes, particularly B cells, which produce specific antibodies targeting particular pathogens. Each type of antibody binds to a unique antigen on a pathogen, marking it for destruction or neutralization.

      • Memory Cells: After exposure to a pathogen, some B cells become memory cells, providing long-term immunity by remembering the specific pathogen, allowing faster and more effective responses to subsequent infections.

Specific Immune Response

  1. Pathogens infect a host

  2. Lymphocyte B cells go around trying to attach to the antigen until the right one is found

  3. When a lymphocyte has receptors that match the antigen shape, they bind together.

  4. The lymphocyte then becomes activated, and undergoes mitosis. It clones itself into plasma cells and memory cells.

  5. Plasma cells produce and release antibodies into the bloodstream.

  6. Antibodies stick to the pathogen, stopping them from moving and acting as a marker for phagocytes, helping them engulf and destroy the pathogen.

  7. Memory cells remain in the bloodstream to fight future infection from the same pathogen.

Active Immunity vs Passive Immunity

Active Immunity

  • Definition: Active immunity is the immunity that results from the body's own immune response to pathogens or antigens, involving the production of antibodies.

  • Examples:

    • Vaccination: When a person receives a vaccine, their body generates antibodies against the specific pathogen without causing the disease.

    • Natural infection: When an individual is exposed to a pathogen and their immune system responds by producing antibodies.

  • Duration: Active Immunity provides a slower but long lasting immunity to a pathogen, since memory cells remain in your bloodstream.

Passive Immunity

  • Definition: Passive immunity is the short-term immunity that occurs when antibodies are transferred from one individual to another, providing immediate protection without the host's immune system having to produce the antibodies itself.

  • Examples: This can occur naturally, such as when a mother passes antibodies to her baby through breast milk, or artificially, through the administration of antibody-containing blood products.

  • Duration: Passive immunity provides immediate protection but only lasts for a limited time, typically weeks to months, as the transferred antibodies are eventually broken down by the recipient's body.

Vaccination

  • Vaccination Process:

    • Introduction of weakened or inactive pathogens into the body.

    • Lymphocytes respond, producing antibodies.

    • Memory cells form to provide long-term immunity.

    Role of Vaccination: Vaccination plays a crucial role in controlling the spread of diseases, including disease eradication programs (e.g., smallpox). By stimulating the immune system, vaccines prepare the body to recognize and fight pathogens without causing the disease itself.

Viruses

  • Definition: Viruses are pathogens that are not considered living organisms. They require host cells to reproduce and sustain themselves.

  • Structure:

    • Consist of genetic material (either DNA or RNA) encased in a protein coat (capsid).

    • Some viruses have an outer lipid envelope.

  • Functions:

    • Viruses infect host cells by attaching to specific receptors on the cell surface.

    • Once inside, they hijack the host's cellular machinery to replicate their genetic material and produce new virus particles.

Antibiotics

  • Definition: Antibiotics are drugs that modify or affect chemical reactions in the body to inhibit the growth of or kill bacteria.

  • Structure: Varies widely among different types of antibiotics but typically consist of specific molecular structures designed to target bacterial cells.

  • Functions:

    • Antibiotics target bacterial infections, disrupting critical processes such as cell wall synthesis, protein synthesis, or DNA replication.

    • They are ineffective against viruses, as viruses do not have cellular structures that antibiotics can target.

  • Their effectiveness is also influenced by factors such as the presence of antibiotic-resistant strains and the ability of the antibiotic to reach the site of infection.