Microbiology & Immunology Review: Key Concepts and Clinical Applications

Part 1: The Invisible Invaders

Viruses, Viroids, and Prions

Viruses, viroids, and prions are unique infectious agents that differ fundamentally from cellular life forms. Understanding their structure and mechanisms is essential for microbiology students.

• Viruses: Non-living entities composed of a capsid (protein coat) and genetic material (either DNA or RNA, never both). They lack metabolism and cannot reproduce independently, requiring a host cell for replication.

• Viroids: Small, circular pieces of infectious RNA that infect plants. Unlike viruses, viroids lack a protein coat.

• Prions: Infectious proteins with no nucleic acids. Prions cause disease by inducing abnormal folding of normal proteins, particularly in the brain, leading to neurodegeneration.

Example: Prion diseases include Creutzfeldt-Jakob disease in humans and mad cow disease in cattle.

Part 2: Infection and the "Secret War"

Normal Microbiota and Lines of Defense

The human body hosts a diverse microbiota acquired after birth. The immune system protects against pathogens through multiple lines of defense.

• Normal Microbiota: Microorganisms that colonize the body soon after birth, providing protection and aiding in health.

• First Line (Innate Immunity): Physical barriers (e.g., skin) and chemical defenses (e.g., lysozyme in tears) prevent pathogen entry.

• Second Line (Innate Immunity): Cellular defenses (phagocytes), inflammation, and fever (mediated by pyrogens) respond to invaders.

• Third Line (Adaptive Immunity): Specific responses involving B cells (produce antibodies for humoral immunity) and T cells (cell-mediated immunity targeting infected or cancerous cells).

Antibodies (MADGE)

Antibodies are immunoglobulins classified as IgM, IgA, IgD, IgG, and IgE. IgG is the most abundant in blood and can cross the placenta.

• IgG: Most common; crosses placenta.

• IgM: First produced in response to infection.

• IgA: Found in mucosal areas and secretions.

• IgD: Functions mainly as a receptor on B cells.

• IgE: Involved in allergic responses.

Part 3: Immune Failures and HIV

Vaccines and Hypersensitivities

Vaccines stimulate immunity, while hypersensitivity reactions represent failures or overreactions of the immune system.

• Vaccines: Provide active immunity via exposure to antigens. Types include:

  • Attenuated: Live but weakened pathogens.

  • Inactivated: Killed pathogens.

  • Toxoid: Inactivated toxins.

• Hypersensitivities (ACID Mnemonic):

  • Type I (Immediate): Allergies, anaphylaxis.

  • Type II (Cytotoxic): Antibody-mediated cell destruction (e.g., transfusion reactions).

  • Type III (Immune Complex): Immune complexes deposit in tissues (e.g., lupus).

  • Type IV (Delayed): T-cell mediated (e.g., poison ivy, TB skin test).

HIV/AIDS

• HIV: Infects and destroys Helper T cells (CD4+).

• AIDS Diagnosis: CD4+ T cell count < 200 cells/µL or presence of AIDS-defining opportunistic infections.

Part 4: Clinical Tour

Microbial Diseases by System

Microbial diseases affect various organ systems, each with characteristic pathogens and clinical presentations.

Skin (Ch 19)

• Staphylococcus aureus: Causes impetigo and other skin infections.

• Streptococcus pyogenes: Can cause necrotizing fasciitis ("flesh-eating" disease).

Nervous System (Ch 20)

• Streptococcus pneumoniae: Leading cause of adult meningitis.

• Botulism: Causes flaccid paralysis.

• Tetanus: Causes muscle contraction ("lockjaw").

• Rabies: Zoonotic virus; fatal once symptoms appear.

Cardiovascular (Ch 21)

• Lyme Disease: Caused by Borrelia burgdorferi, transmitted by deer ticks; "bull’s-eye" rash.

• Malaria: Protozoan infection (Plasmodium spp.), transmitted by mosquitoes; destroys red blood cells.

Respiratory (Ch 22)

• Tuberculosis: Caused by Mycobacterium tuberculosis; resists immune killing due to mycolic acid cell wall.

• Diphtheria: Forms thick pseudomembrane in airway.

Digestive (Ch 23)

• Helicobacter pylori: Main cause of peptic ulcers.

• Hepatitis A & E: Spread by fecal-oral route.

• Hepatitis B & C: Spread via blood or sexual contact.

Urinary/Reproductive (Ch 24)

• Escherichia coli: Most common cause of urinary tract infections (UTIs).

• Gonorrhea: Causes painful discharge.

• Syphilis: Has four distinct clinical phases.

• Chlamydia: Most common reportable sexually transmitted disease (STD).

Summary Table: Hypersensitivity Types

Type	Mechanism	Examples
I (Immediate)	IgE-mediated, mast cell activation	Allergies, anaphylaxis
II (Cytotoxic)	IgG/IgM antibodies target cells	Transfusion reactions
III (Immune Complex)	Immune complexes deposit in tissues	Lupus, rheumatoid arthritis
IV (Delayed)	T-cell mediated	Poison ivy, TB skin test

Additional info:

• Viruses are classified based on their nucleic acid type (DNA or RNA), capsid symmetry, and presence or absence of an envelope.

• Prion diseases are unique in that they are not caused by conventional pathogens but by misfolded proteins.

• Adaptive immunity involves memory cells, allowing for faster and stronger responses upon re-exposure to the same pathogen.

• Vaccines are a cornerstone of public health, preventing diseases by priming the immune system.