Clinical Microbiology Exam 3 Study Notes: Immunity, Antimicrobial Drugs, and Vaccines

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Immunity and Immune Responses

Innate vs Adaptive Immunity

The immune system protects the body from pathogens through two main types of defense: innate (nonspecific) and adaptive (specific) immunity.

Innate Immunity: Present at birth; provides immediate, nonspecific defense against pathogens.
Adaptive Immunity: Develops after exposure to antigens; provides specific, long-lasting protection.
Key Differences: Innate immunity responds rapidly and does not improve with repeated exposure, while adaptive immunity has memory and improves upon subsequent exposures.
Example: Skin and mucous membranes are part of innate immunity; antibodies produced after vaccination are part of adaptive immunity.

Physical and Chemical Factors in Immunity

Physical and chemical barriers are the first line of defense in innate immunity, preventing pathogen entry and growth.

Physical Factors: Skin, mucous membranes, cilia, and tears physically block or remove microbes.
Chemical Factors: Lysozyme in saliva and tears, acidic pH of the stomach, and antimicrobial peptides inhibit or destroy pathogens.
Example: Stomach acid destroys ingested bacteria; lysozyme breaks down bacterial cell walls.

Differential White Blood Cell Count

A differential white blood cell (WBC) count measures the percentages of different types of leukocytes in the blood, which can indicate various health conditions.

High WBC Count: May indicate infection, inflammation, or leukemia.
Low WBC Count: May suggest immunosuppression, bone marrow disorders, or certain infections.
Altered Levels: Elevated neutrophils suggest bacterial infection; increased lymphocytes may indicate viral infection; high eosinophils can point to parasitic infection or allergies.
Example: A patient with high neutrophil count may have a bacterial infection.

Inflammation

Inflammation is a localized response to infection or injury, characterized by redness, heat, swelling, and pain.

Purpose: To contain and eliminate pathogens, remove damaged tissue, and initiate repair.
Key Steps: Vasodilation, increased permeability of blood vessels, migration of phagocytes, and tissue repair.
Example: Swelling and redness around a cut are signs of inflammation.

Antibody Characteristics and Classes

Antibody Structure and Function

Antibodies (immunoglobulins) are proteins produced by B cells that specifically bind antigens to neutralize or mark them for destruction.

Structure: Y-shaped molecules with variable regions for antigen binding.
Function: Neutralization, opsonization, complement activation, and agglutination.

Antibody Classes

There are five main classes of antibodies, each with distinct roles:

Class	Main Function	Location
IgG	Most abundant; crosses placenta; long-term immunity	Blood, extracellular fluid
IgM	First antibody produced; effective in agglutination	Blood, lymph
IgA	Protects mucosal surfaces	Secretions (tears, saliva, mucus)
IgD	Functions as B cell receptor	B cell surface
IgE	Involved in allergic responses and defense against parasites	Bound to mast cells, basophils

Primary vs Secondary Immune Response

Characteristics of Primary and Secondary Responses

The immune system responds differently to first and subsequent exposures to an antigen.

Primary Response: Occurs after first exposure; slower, with lower antibody levels (mainly IgM).
Secondary Response: Faster and stronger due to memory cells; higher antibody levels (mainly IgG).
Example: Booster vaccines elicit a secondary response, resulting in rapid antibody production.

B Cells vs T Cells

Roles in Immunity

B cells and T cells are lymphocytes with distinct functions in adaptive immunity.

B Cells: Produce antibodies (humoral immunity); mature in bone marrow.
T Cells: Mediate cellular immunity; mature in thymus; include helper, cytotoxic, and regulatory T cells.
Example: B cells neutralize toxins; cytotoxic T cells kill virus-infected cells.

Humoral vs Cellular Immunity

Comparison Table

Humoral and cellular immunity are two branches of adaptive immunity, each targeting different types of pathogens.

Feature	Humoral Immunity	Cellular Immunity
Main Cells	B cells	T cells
Effector Molecules	Antibodies	Cytokines, cytotoxic granules
Targets	Extracellular pathogens (bacteria, toxins)	Intracellular pathogens (viruses, some bacteria), cancer cells
Memory	Yes	Yes

Active vs Passive Immunity

Types and Examples

Immunity can be acquired actively or passively, and either naturally or artificially.

Type	How Acquired	Example
Active, Natural	Infection	Recovery from measles
Active, Artificial	Vaccination	MMR vaccine
Passive, Natural	Maternal antibodies	IgG crossing placenta
Passive, Artificial	Injection of antibodies	Antivenom for snakebite

Vaccines

Vaccine Types

Vaccines stimulate adaptive immunity and can be classified by their composition.

Live Attenuated: Weakened form of the pathogen (e.g., MMR vaccine).
Inactivated: Killed pathogen (e.g., polio vaccine).
Subunit: Purified antigenic components (e.g., hepatitis B vaccine).
Toxoid: Inactivated toxins (e.g., tetanus vaccine).
mRNA: Encodes antigenic proteins (e.g., COVID-19 mRNA vaccines).

Vaccine Importance to a Community

Vaccines protect individuals and contribute to herd immunity, reducing disease spread and protecting vulnerable populations.

Herd Immunity: When a high percentage of the community is immune, disease transmission is interrupted.
Example: Widespread measles vaccination prevents outbreaks.

Antimicrobial Drugs

Mechanisms of Action, Side Effects, and Target Microbes

Antimicrobial drugs are used to treat infections caused by bacteria, viruses, fungi, protozoa, and helminths. Understanding their mechanisms, side effects, and spectrum is essential in clinical microbiology.

Drug	Mechanism of Action	Side Effects	Target Microbe
Bacitracin	Inhibits cell wall synthesis (interferes with peptidoglycan transport)	Nephrotoxicity, contact dermatitis	Gram-positive bacteria
Polymyxin B	Disrupts cell membrane integrity	Neurotoxicity, nephrotoxicity	Gram-negative bacteria
Neomycin	Inhibits protein synthesis (binds 30S ribosomal subunit)	Ototoxicity, nephrotoxicity	Broad spectrum (mainly Gram-negative)
Penicillin	Inhibits cell wall synthesis (blocks transpeptidase)	Allergic reactions, GI upset	Gram-positive bacteria
Tamiflu	Neuraminidase inhibitor (prevents viral release)	Nausea, vomiting	Influenza viruses
Ivermectin	Paralyzes parasite by binding glutamate-gated chloride channels	GI upset, dizziness	Helminths
Paxlovid (nirmatrelvir)	Protease inhibitor (blocks viral replication)	Altered taste, diarrhea	SARS-CoV-2 (COVID-19)
Miconazole	Inhibits ergosterol synthesis (disrupts fungal membrane)	Skin irritation	Fungi
Acyclovir	Inhibits viral DNA polymerase	Renal toxicity (rare)	Herpesviruses
Artemisinin	Generates free radicals in parasite	GI upset, headache	Protozoa (Plasmodium spp.)
Chloramphenicol	Inhibits protein synthesis (binds 50S ribosomal subunit)	Aplastic anemia	Broad spectrum
Amoxicillin	Inhibits cell wall synthesis	Allergic reactions, diarrhea	Broad spectrum
Tetracycline	Inhibits protein synthesis (binds 30S ribosomal subunit)	Photosensitivity, teeth discoloration	Broad spectrum
Remdesivir	Inhibits viral RNA-dependent RNA polymerase	GI upset, liver enzyme elevation	SARS-CoV-2 (COVID-19)
Humira	Monoclonal antibody against TNF-α (immunosuppressant)	Increased infection risk	Autoimmune diseases (not antimicrobial)

Additional info: Humira is included for completeness, though it is primarily used for autoimmune diseases, not as an antimicrobial.

Antibiotic Targets and Resistance

Antibiotic Targets: Cell wall synthesis, protein synthesis, nucleic acid synthesis, cell membrane integrity, metabolic pathways.
Resistance Mechanisms: Enzymatic degradation (e.g., beta-lactamases), altered target sites, efflux pumps, reduced permeability.
Example: Methicillin-resistant Staphylococcus aureus (MRSA) has altered penicillin-binding proteins.

Summary Table: Immunity and Antimicrobial Drugs

Topic	Key Points
Innate Immunity	Immediate, nonspecific, no memory
Adaptive Immunity	Specific, memory, improves with exposure
Antibody Classes	IgG, IgM, IgA, IgD, IgE
Vaccine Types	Live attenuated, inactivated, subunit, toxoid, mRNA
Antimicrobial Drugs	Target cell wall, protein synthesis, nucleic acids, membranes
Resistance	Enzymatic inactivation, target modification, efflux