Microbiology: Immunology, Antimicrobial Drugs, and Infectious Diseases – Study Guide

Immunology and Vaccines

Definitions and Types of Vaccines

Vaccines are biological preparations that provide acquired immunity to a particular infectious disease. They typically contain an agent resembling a disease-causing microorganism, often made from weakened or killed forms of the microbe, its toxins, or one of its surface proteins.

• Attenuated vaccines: Contain live microorganisms that have been weakened so they cannot cause disease in healthy people. They often provide strong, long-lasting immunity.

• Inactivated vaccines: Contain microorganisms that have been killed. They are safer but may require booster shots.

• Subunit, toxoid, VLP (virus-like particle), and conjugated vaccines: Contain only parts of the microorganism or inactivated toxins, often combined with other molecules to enhance immune response.

Example: The Sabin polio vaccine is an oral, live attenuated vaccine, while the Salk polio vaccine is an injected, inactivated vaccine.

Principles of Vaccination

• Vaccination: The process of stimulating the immune system to develop adaptive immunity to a pathogen, usually by administration of a vaccine.

• Herd immunity: When a large portion of a community becomes immune to a disease, making its spread unlikely.

Key Point: Vaccination works by exposing the immune system to antigens, prompting the production of memory cells that respond rapidly upon future exposure.

Serological Testing and Immunoassays

Serological tests detect antibodies or antigens in a sample, helping diagnose infections and immune responses.

• Precipitation reactions: Involve the formation of a visible precipitate when antibodies bind to soluble antigens.

• Agglutination reactions: Involve the clumping of particles (e.g., cells, latex beads) when antibodies bind to antigens on their surfaces.

• Hemagglutination: Agglutination of red blood cells, used in blood typing and some viral diagnostics.

• Neutralization tests: Measure the ability of antibodies to neutralize pathogens or their toxins.

• Complement-fixation tests: Detect the presence of specific antibodies by their ability to fix complement in the presence of antigen.

• ELISA (Enzyme-Linked Immunosorbent Assay): Detects and quantifies antigens or antibodies using enzyme-linked antibodies and a colorimetric substrate.

Example: Direct ELISA detects antigens; indirect ELISA detects antibodies.

Hypersensitivity and Autoimmunity

Types of Hypersensitivity

• Localized vs. systemic anaphylaxis: Localized affects a specific area (e.g., hay fever); systemic can be life-threatening (e.g., anaphylactic shock).

• Desensitization: Gradual exposure to increasing amounts of allergen to reduce sensitivity.

• Cytotoxic reactions: Antibodies target cells, leading to cell destruction (e.g., blood transfusion reactions).

• Immune complex reactions: Antigen-antibody complexes deposit in tissues, causing inflammation.

• Delayed-type hypersensitivity: T-cell mediated, occurs hours to days after exposure (e.g., contact dermatitis).

Autoimmune Diseases and Transplantation

• Autoimmunity: Immune system attacks self-antigens (e.g., Graves' disease).

• Transplant rejection: Immune response against transplanted tissue, often due to differences in MHC (major histocompatibility complex) antigens.

• Immunosuppressive therapy: Used to prevent rejection but increases risk of infection.

HIV and AIDS

HIV Infection and Immune Evasion

• HIV (Human Immunodeficiency Virus): Infects CD4+ T cells, leading to immune deficiency (AIDS).

• Immune evasion: HIV mutates rapidly, hides in host cells, and destroys immune cells.

• Transmission: Primarily through blood, sexual contact, and from mother to child.

• Prevention: Safe sex, needle exchange, antiretroviral therapy.

Antimicrobial Drugs

Types and Mechanisms of Action

• Antibiotics: Substances produced by microorganisms that inhibit or kill other microbes.

• Spectrum of activity: Broad-spectrum antibiotics act against a wide range of bacteria; narrow-spectrum act against specific types.

• Mechanisms: Inhibit cell wall synthesis, protein synthesis, nucleic acid synthesis, or disrupt membranes.

• Antiviral drugs: Inhibit viral replication (e.g., acyclovir for herpesviruses).

• Antifungal drugs: Target fungal cell membranes or cell wall synthesis.

Example: Penicillin's inhibit bacterial cell wall synthesis; tetracyclines inhibit protein synthesis.

Drug Resistance

• Resistance: Microbes evolve mechanisms to evade drug action (e.g., enzymatic degradation, efflux pumps, target modification).

• Synergism and antagonism: Drug combinations can enhance (synergism) or reduce (antagonism) effectiveness.

Central Nervous System Infections

Meningitis and Encephalitis

• Meningitis: Inflammation of the meninges, caused by bacteria (e.g., Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumoniae), viruses, or fungi.

• Encephalitis: Inflammation of the brain, often viral.

• Diagnosis: Analysis of cerebrospinal fluid (CSF).

Diseases of the Respiratory System

Upper and Lower Respiratory Tract Infections

• Common diseases: Pharyngitis, laryngitis, tonsillitis, sinusitis, otitis media, diphtheria, pertussis, tuberculosis, pneumonia, influenza.

• Symptoms: Cough, sore throat, fever, difficulty breathing.

• Diagnosis: Clinical symptoms, laboratory tests, and sometimes imaging.

Diseases of the Urinary and Reproductive Systems

Urinary Tract Infections (UTIs)

• Common pathogens: Escherichia coli is the most frequent cause.

• Symptoms: Painful urination, frequent urge to urinate, cloudy urine.

• Transmission: Often from the patient's own intestinal microbiota.

Sexually Transmitted Infections (STIs)

• Bacterial STIs: Gonorrhea, syphilis, chlamydia.

• Viral STIs: Genital herpes, human papillomavirus (HPV), HIV.

• Fungal STIs: Candidiasis (yeast infection).

• Protozoal STIs: Trichomoniasis.

Example: Trichomonas vaginalis causes trichomoniasis, a protozoal infection of the reproductive tract.

Sample Table: Types of Vaccines

Type	Example	Key Features
Attenuated (live)	MMR, Sabin polio	Strong, long-lasting immunity; risk in immunocompromised
Inactivated (killed)	Salk polio, influenza	Safer, but may require boosters
Subunit/conjugate	Hepatitis B, Hib	Contains only parts of pathogen; fewer side effects
Toxoid	Tetanus, diphtheria	Inactivated toxins; protects against toxin-mediated disease

Key Equations

• Antibody titer calculation:

$\text{Titer} = \frac{1}{\text{highest dilution showing a positive reaction}}$

• Drug effectiveness (MIC):

$\text{MIC} = \text{Minimum Inhibitory Concentration}$

Summary

This study guide covers key concepts in immunology, antimicrobial drugs, and infectious diseases, including vaccine types, immune responses, serological testing, hypersensitivity, autoimmunity, HIV/AIDS, mechanisms of antimicrobial action and resistance, and major infectious diseases of the nervous, respiratory, urinary, and reproductive systems. Understanding these principles is essential for diagnosing, treating, and preventing infectious diseases in clinical practice.