Microbiology Study Notes: Infectious Diseases and Immunity (Chapters 14–16)

Chapter 14: Infectious Diseases

Classification of Infectious Diseases

Infectious diseases can be classified based on their duration, recurrence, and relationship to other diseases.

• Acute Disease: Develops rapidly but lasts a short time (e.g., influenza).

• Chronic Disease: Develops slowly and persists over a long period (e.g., tuberculosis).

• Latent Disease: Pathogen remains inactive for a time but can reactivate (e.g., herpes simplex virus).

• Primary Infection: The initial infection in a healthy individual.

• Secondary Infection: Occurs when a second pathogen infects a host weakened by a primary infection (e.g., bacterial pneumonia following influenza).

Signs vs Symptoms

• Signs: Objective changes that can be observed or measured by others (e.g., fever, rash).

• Symptoms: Subjective changes experienced by the patient (e.g., pain, fatigue).

Endemic, Epidemic, and Pandemic

• Endemic: Disease constantly present in a population (e.g., malaria in certain regions).

• Epidemic: Sudden increase in disease cases above normal expectancy in a region (e.g., Ebola outbreak).

• Pandemic: An epidemic that spreads across countries or continents (e.g., COVID-19).

Stages of Infectious Disease

1. Incubation Period: Time between pathogen entry and appearance of symptoms.

2. Prodromal Period: Early, mild symptoms appear.

3. Period of Illness: Disease is most severe; signs and symptoms are evident.

4. Period of Decline: Signs and symptoms subside.

5. Period of Convalescence: Recovery; body returns to pre-disease state.

Modes of Transmission

• Biological Transmission: Pathogen develops or multiplies within a vector (e.g., malaria in mosquitoes).

• Mechanical Transmission: Passive transport of pathogens on the body of a vector (e.g., flies carrying bacteria on their feet).

Virulence Factors

Virulence factors are molecules produced by pathogens that contribute to their ability to cause disease.

• Adhesins: Enable attachment to host cells.

• Enzymes: Such as hyaluronidase, which break down host tissues.

• Toxins: Exotoxins and endotoxins that damage host cells.

• Capsules: Prevent phagocytosis by host immune cells.

Chapter 15: Innate Immunity

Physical and Chemical Factors of Innate Immunity

Innate immunity provides the first line of defense against pathogens through physical and chemical barriers.

• Physical Barriers: Skin, mucous membranes, cilia, and tears prevent pathogen entry.

• Chemical Barriers: Lysozyme in saliva and tears, acidic pH of stomach, and antimicrobial peptides.

Inflammation and Its Role

• Definition: Localized response to tissue injury or infection, characterized by redness, heat, swelling, and pain.

• Role: Contains infection, removes pathogens, and initiates tissue repair.

Fever and Its Role

• Definition: Systemic increase in body temperature, often due to pyrogens.

• Role: Inhibits pathogen growth and enhances immune responses.

Complement System Activation and Outcomes

• Activation Pathways: Classical, alternative, and lectin pathways.

• Outcomes:

  • Opsonization (enhanced phagocytosis)

  • Cell lysis (via membrane attack complex)

  • Inflammation

Key Equation:

$\text{C3} \xrightarrow{\text{cleavage}} \text{C3a} + \text{C3b}$

Additional info: The complement system is a group of serum proteins that work in a cascade to destroy pathogens.

Chapter 16: Adaptive Immunity

Humoral vs Cellular Immunity

Adaptive immunity is divided into humoral and cellular branches, each with distinct roles.

• Humoral Immunity: Mediated by B cells and antibodies; defends against extracellular pathogens.

• Cellular Immunity: Mediated by T cells; targets infected or abnormal cells.

Role and Function of B and T Cells

• B Cells: Produce antibodies, present antigens, and differentiate into plasma and memory cells.

• T Cells: Include helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells; coordinate immune responses and kill infected cells.

Immunoglobulins and Their Role in Immunity

Immunoglobulins (antibodies) are proteins produced by B cells that recognize and neutralize antigens.

• IgG: Most abundant; crosses placenta; provides long-term immunity.

• IgM: First antibody produced in response to infection.

• IgA: Found in mucosal areas and secretions.

• IgE: Involved in allergic responses and defense against parasites.

• IgD: Functions mainly as a B cell receptor.

MHC I and II

• MHC I: Present on all nucleated cells; present endogenous antigens to CD8+ cytotoxic T cells.

• MHC II: Present on antigen-presenting cells; present exogenous antigens to CD4+ helper T cells.

Clonal Selection and Deletion

• Clonal Selection: Activation and proliferation of lymphocytes specific to an antigen.

• Clonal Deletion: Elimination of self-reactive lymphocytes to prevent autoimmunity.

Immunological Memory

• Memory B and T cells persist after initial exposure, enabling a faster and stronger response upon re-exposure to the same antigen.

Types of Adaptive Immunity

Type	Natural	Artificial
Active	Infection (body produces antibodies)	Vaccination (body produces antibodies)
Passive	Maternal antibodies (placenta, breast milk)	Antibody injection (e.g., antitoxin)

Additional info: Active immunity involves the host's own immune response, while passive immunity involves transfer of antibodies from another source.