Pathophysiology of Infectious Diseases

Pathophysiology of Infectious Diseases - Week Three Notes

Introduction to Infectious Diseases

• Focus on infectious diseases as related to the immune system.

• Objectives:

  • Review the immune system (pathology side).

  • Discuss terminology and basic microbiology.

  • Cover type II hypersensitivity reactions and graft versus host reactions.

  • Explore the pathology of malaria and AIDS.

• Prepare to discuss:

  • Disease processes.

  • How these diseases affect the immune system and vice versa.

  • Review notes on disease characteristics: transmission, prevention, treatment, and diagnosis.

Overview of the Immune System

• Two types of immunity:

  • Innate Immunity (non-specific defense)

  • Involves barriers (e.g., skin, mucosa).

  • The skin is the largest organ and first line of defense against pathogens.

  • Includes patrolling cells:

    • Monocytes, Macrophages, and Neutrophils - roam looking for invaders.

    • Respond to injurious stimuli and signal for help if necessary.

  • Adaptive Immunity (specific defense)

  • Involves specifically trained immune cells.

  • The thymus is central for training T-cells.

  • Key cells:

    • Memory B cells (remember infections for faster response),

    • Plasma cells (produce antibodies like IgG).

    • Cytotoxic T cells (destroy infected cells).

Complement System

• A cascade of proteins crucial for innate immunity.

• Functions:

  • Detect and label pathogens (similar to Velcro).

  • Can lyse pathogens through the formation of a membrane attack complex (C1 to C9).

• Produces holes in pathogen cell walls, leading to cell death.

Phagocytosis Process

• Macrophage Mechanism: Macrophages eat bacteria, breaking them down using lysosomes.

• Fragments signal to B-cells and T-cells aiding in adaptive immunity.

Types of Antibodies

• Five Classes of Antibodies: IgA, IgD, IgE, IgG, IgM.

• Key Points:

  • IgG: Most common, important for vaccinations.

  • IgE: Associated with allergies; triggers inflammation when interacting with allergens.

  • IgA: Found in secretions (e.g., breast milk).

  • IgM: First responder, but less effective.

Cellular Differentiation and Immune Response

• Stem Cells’ Pathway: Differentiate into various immune cells based on signals received.

• Important cells:

  • Dendritic Cells: Present invaders to T-cells and B-cells.

  • Natural Killer Cells: Responsible for apoptosis of infected cells.

Lymphatic System

• Lymph nodes are critical for gathering immune cells during infection.

• Site of communication among macrophages, neutrophils, T and B cells.

Immune System Malfunctions

• Congenital Immunodeficiency: Born without a functioning immune system.

• Immunodeficiency: Can develop later due to infections or medications.

• Hypersensitivity: Overactive immune responses to benign substances like pollen; causes allergies and inflammation.

• Transplantation Rejection: Immune response against foreign tissue.

• Autoimmune Disorders: Immune system mistakenly attacks the body.

Type II Hypersensitivity (Cytotoxic Reaction)

• Mediated by IgG and IgM antibodies.

• Common Example: Reactions during blood transfusions.

  • Red blood cells mistaken as foreign agents leading to complement activation & destruction.

  • Importance of matching blood types:

  • O is a universal donor.

  • A and B types match according to their antigens and antibodies.

• Rh Factor: Presence (+) or absence (-) must match in blood transfusions.

Graft Versus Host Disease

• Two scenarios:

  • Donated tissue has its own immune cells attacking host.

  • Host immune cells attacking donated tissues, leading to defense complications.

• Prevention Strategies:

  • Ensure blood type compatibility.

  • Use of steroids to manage the immune response.

  • Plasmapheresis: Exchange of immune cells to prevent rejection.

Type III Hypersensitivity (Immune Complex Mediated Disorders)

• Mechanism: Immune complexes form (antigen-antibody) and deposit in tissues, invoking inflammation (e.g., Stevens-Johnson syndrome caused by vancomycin).

Infectious Disease Pathophysiology

Mechanism of Disease Development

• Key Elements: Host, environment, and infectious agent (microbe).

• Infection process: Agent enters host and causes disease, possibly re-entering the environment.

Types of Infectious Agents

1. Prions: Misfolded proteins leading to neurodegeneration (e.g., Mad Cow Disease).

2. Viruses: Simple organisms requiring a host to replicate (DNA/RNA encapsulated in protein).

3. Bacteria: More complex, can live outside the host.

4. Fungi: Eukaryotic organisms with cell walls made of chitin.

5. Parasites: Require a host organism to survive and replicate (e.g., malaria).

Inflammation vs Immunological Response

• Inflammation: Localized; symptoms include pain, swelling, redness, heat, and loss of function.

• Immunological Response: Systemic involving memory and adaptive immune cells; often includes fever.

• Mediators of Inflammation:

  • Histamine: Causes vasodilation.

  • Leukotrienes: Slow-acting, promoting long-term inflammation.

  • Cytokines: Proteins signaling immune response.

Sepsis

Pathophysiology

• Uncontrolled inflammatory response to a disseminated infection resulting in shock and possible organ failure.

• Signs: Fever, tachycardia, hypotension, increased lactate levels, leukocytosis.

Treatment Strategy

• Antibiotics: Essential for managing bacterial infections causing sepsis.

• IV Fluids: To combat hypotension and support circulation.

Malaria

Overview

• Caused by the Plasmodium parasite, transmitted by Anopheles mosquitoes.

Life Cycle

1. Mosquito bites (injects parasite).

2. Parasite infects liver (hepatocytes).

3. Parasite enters red blood cells, replicates, and causes hemolysis.

Symptoms

• Common Symptoms: Fever, chills, body aches, fatigue, nausea, and vomiting; jaundice in advanced cases.

Treatment

• Prevention: Prophylactic medications (e.g., methylquin hydrochloride) and mosquito control measures.

HIV and AIDS

Overview

• HIV: Human Immunodeficiency Virus; leads to AIDS (Acquired Immune Deficiency Syndrome).

Pathophysiology

• HIV attacks CD4 T-cells, leading to immunodeficiency and increased susceptibility to opportunistic infections.

Transmission

• Via blood, sexual intercourse, and mother-to-child.

Diagnosis

• Tests: ELISA test and Western blot for antibodies/antigens.

Treatment

• Focus on prevention, safe sex practices, and medications such as antiretroviral therapy (ART) to suppress viral load.

Conclusion

• Understanding the immune mechanisms in infectious diseases helps in effectively diagnosing and treating diseases like malaria and HIV/AIDS.

• Importance of continued education and public health interventions to manage infectious diseases.