Pathogen Mechanisms of Disease

How Microorganisms Cause Harm to the Host

Different classes of microorganisms employ distinct strategies to inflict damage on their hosts. Understanding these mechanisms is crucial for comprehending disease progression and host responses.

• Fungi:

  • Produce toxic metabolic products that can damage host tissues.

  • Can provoke allergic responses in susceptible individuals.

  • Some secrete toxins that inhibit host protein synthesis.

  • Release proteases, which modify and compromise host cell membranes.

  • Many possess capsules, aiding in resistance to phagocytosis by immune cells.

  • Frequently cause topical (surface) infections.

• Protozoa:

  • Single-celled eukaryotes that cause disease symptoms by digesting host cells.

  • Exhibit sophisticated mechanisms to evade host immune defenses.

• Helminths (Worms):

  • Multicellular parasites that utilize host tissues for growth and development.

  • Cause physical and cellular damage due to their size and movement.

  • Release waste products that can be toxic to the host.

• Algae:

  • Single-celled organisms, some of which produce potent neurotoxins.

Example: Aspergillus species (fungi) can cause allergic bronchopulmonary aspergillosis, while Plasmodium (protozoa) digests red blood cells in malaria.

Portals of Entry and Exit

Routes of Microbial Transmission

Microorganisms enter and exit the human body through specific anatomical routes, facilitating infection and transmission.

• Respiratory Tract: Entry and exit via inhalation or exhalation, coughing, and sneezing.

• Gastrointestinal (GI) Tract: Entry through ingestion; exit via feces and saliva.

• Genitourinary (GU) Tract: Entry and exit through urine and reproductive secretions.

• Skin: Direct penetration or exit through wounds or lesions.

• Blood: Entry and exit via wounds, insect bites, or medical procedures.

• Arthropods: Insects and related organisms act as vectors, transmitting pathogens between hosts.

Example: The influenza virus exits the respiratory tract via droplets expelled during coughing or sneezing.

Innate vs. Adaptive Immunity

Overview of the Body's Defense Systems

The immune system is divided into two main branches: innate (nonspecific) and adaptive (specific) immunity. Each plays a distinct role in host defense.

• Susceptibility: Defined as the lack of resistance to disease.

• Innate Immunity:

  • Immediate, nonspecific defense against a broad range of pathogens.

  • Always present and ready to act; rapid response.

  • No memory component—responds the same way to repeated exposures.

  • Key components: physical barriers (skin, mucous membranes), antimicrobial substances, inflammation, fever, and phagocytes.

• Adaptive Immunity:

  • Highly specific response to particular pathogens.

  • Slower initial response but develops immunological memory for faster, stronger reactions upon re-exposure.

  • Includes humoral immunity (antibody-mediated) and cellular immunity (cell-mediated).

Example: Vaccination induces adaptive immunity, enabling rapid antibody production upon future exposure to the same pathogen.

Specific Components and Signaling of Innate Immunity

Key Molecules and Cellular Responses

Innate immunity relies on molecular recognition and signaling to mount effective defenses against pathogens.

• Interferons (Alpha & Beta):

  • Proteins produced by virus-infected cells.

  • Act as alarm signals to neighboring cells, inducing antiviral states and inhibiting viral replication.

• Toll-like Receptors (TLRs):

  • Pattern recognition receptors on immune cells that detect PAMPs (Pathogen-Associated Molecular Patterns).

  • Examples of PAMPs: LPS (lipopolysaccharide) from Gram-negative bacteria, flagellin from bacterial flagella, peptidoglycan from bacterial cell walls.

  • TLR activation triggers the release of cytokines—signaling molecules that recruit and activate other immune cells.

Example: Interferon production during viral infection helps limit the spread of viruses to uninfected cells.

White Blood Cell (WBC) Count

Diagnostic Significance in Infection

White blood cell counts are a key diagnostic tool for assessing immune status and identifying infections.

• High WBC Count: May indicate bacterial infections or autoimmune diseases.

• Low WBC Count: Often associated with viral infections (e.g., viral pneumonia) or conditions that suppress the immune system.

• Normal Range: cells per microliter ().

Example: A patient with bacterial meningitis may present with leukocytosis, while a patient with influenza may have leukopenia.

First Line of Defense: Physical and Chemical Factors (Innate Immunity)

Barriers Preventing Microbial Entry

The body's first line of defense consists of physical and chemical barriers that prevent pathogen entry and colonization.

• Physical Factors:

  • Skin:

    • Epidermis: Outermost layer; tightly packed epithelial cells with keratin for toughness.

    • Dermis: Inner layer; composed of connective tissue.

  • Mucous Membranes: Secrete mucus to trap microbes and prevent drying; cilia move trapped microbes out of the body.

  • Other Mechanical Defenses: Earwax, vaginal secretions, vomiting, defecation, diarrhea, and peristalsis physically expel microbes.

• Chemical Factors:

  • Sebum: Oily secretion from skin glands; maintains acidic pH () to inhibit microbial growth.

  • Lysozyme: Enzyme in perspiration, tears, saliva, and urine; breaks down bacterial cell walls.

  • Gastric Juice: Stomach acid with pH destroys most ingested microbes.

  • Vaginal Secretions: Maintain acidic pH to deter pathogenic microbes.

Example: The acidic environment of the stomach prevents most ingested bacteria from surviving and causing infection.

Additional info: The distinction between innate and adaptive immunity is foundational in immunology. Innate immunity provides immediate, broad-spectrum defense, while adaptive immunity offers long-lasting, specific protection through memory cells and antibodies.