Understanding Infectious Diseases and the Immune System

Posted on Sep 8, 2024 in Biology

Infectious Diseases and Pathogens

Koch’s Postulates

  1. The pathogen must be present in the diseased animal.
  2. The pathogen is isolated and grown in the laboratory.
  3. The pathogen is injected into a healthy organism.
  4. The pathogen from the second animal is isolated and grown in the laboratory. It should be identical to the pathogen from the first animal.

Pathogen Transmission: Pathogens can spread to humans through air, food, water, person-to-person contact, and animal bites.

First Line of Defense

1. Mucous Membranes:

  • Mucus traps pathogens.
  • Lysozymes are enzymes that destroy bacteria by destroying their membranes.

Mucus traps pathogens in the respiratory system, digestive system, urethra, and vagina.

2. Skin:

  • Pores release sweat, oil (which helps remove pathogens), and waxes (in the ear).
  • When pores are clogged by pathogens, oil cannot escape, leading to pimple formation.

Second Line of Defense

A. Inflammatory Response

  • Histamine: A substance that makes capillaries permeable.
  • Phagocytes: Cells that destroy pathogens.
  • Neutrophils: The most abundant type of phagocyte; they can squeeze through capillary walls to reach infections.
  • Macrophages: A type of phagocyte that engulfs pathogens.
  • Natural Killer Cells: White blood cells that attack pathogen-infected cells, not the pathogens themselves.

B. Temperature Response

  • Fever: A rise in body temperature above 37°C, indicating the body’s response to an infection.

C. Proteins

  • Complement System: Made up of 20 different proteins that eliminate infected cells.

Specific Defenses (Third Line of Defense)

The Immune System

The immune system consists of cells and tissues that recognize and attack foreign substances in the body.

Tissues of the Immune System:

  • Bone marrow (produces lymphocytes)
  • Lymph vessels
  • Lymph nodes (contain lymphocytes)
  • Adenoids
  • Tonsils
  • Spleen (largest lymphatic organ, stores red blood cells and produces lymphocytes)
  • Thymus (produces a special type of lymphocyte)

White Blood Cells of the Immune System: Lymphocytes

Types of Lymphocytes:

  • B cells (produced and mature in the bone marrow)
  • T cells (produced in bone marrow but mature in the thymus)

Inflammation

Any tissue damage causes inflammation:

  1. Bacteria enter.
  2. Plasma proteins leak from blood vessels.
  3. Plasma proteins mark the bacteria.
  4. White blood cells engulf the bacteria.

Immune Response

The body’s reaction against an antigen.

Cell-Mediated Response

  1. Macrophages engulf a virus and present viral antigens.
  2. T cells receive the viral antigens and bind to them, causing the macrophage to release interleukin-1.
  3. Interleukin-1 activates helper T cells, which release interleukin-2.
  4. Interleukin-2 stimulates the division of helper T cells and activates cytotoxic T cells.
  5. Cytotoxic T cells destroy infected cells, and some become memory T cells.
  6. Suppressor T cells help shut down the immune response after the pathogen is destroyed.

Humoral Immune Response

  1. Macrophages engulf a virus and present viral antigens.
  2. T cells receive the viral antigens and bind to them, causing the macrophage to release interleukin-1.
  3. Interleukin-1 activates helper T cells, which release interleukin-2.
  4. B cells are activated by the release of interleukin-2 and begin to multiply.
  5. Some become memory B cells, and others become plasma B cells.
  6. Plasma cells produce antibodies that bind to antigens, marking them for destruction.

Primary Immune Response

The first encounter with a disease:

  1. Antigen enters.
  2. Effector cells (killer cells) are produced.
  3. Memory cells are produced (remember the disease but do not attack).

Secondary Immune Response

Subsequent encounters with the same disease:

  1. Antigen enters.
  2. Memory cells reproduce quickly into effector cells (in greater numbers than before) and memory cells (in greater numbers than before).
  3. The pathogen is eradicated more easily. Antibodies are produced faster.

Exceptions: Cold and flu viruses (their external protein coat mutates).

Vaccination

The introduction of antigens into the body to induce immunity. Memory cells remember the vaccine (the antigen), making it easier to eradicate the pathogen during the secondary immune response. Sometimes, vaccines require a booster shot to ensure the immune system recognizes the pathogen. Examples of vaccines include those for polio, measles, and tetanus.

Immune Deficiency

A condition where the immune system does not function properly. Immune deficiency can be acquired through a virus or be present from birth.

Autoimmune Diseases

The immune system attacks the body’s own tissues, mistaking them for invaders. Treatment often involves immunosuppressants to stabilize the immune system. An example is arthritis, where the immune system attacks the joints.

HIV Virus

Enters a host cell and makes a copy of its DNA.

Asthma

A condition where excess mucus lines the airways, making it difficult to breathe.