Unit 3 Study Guide: Innate and Adaptive Immunity, Immune System Disorders, and Laboratory Diagnostics

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Innate Nonspecific Host Defenses

Physical Defenses

Physical defenses are the body's first line of protection against infection, preventing the entry and establishment of pathogens.

Skin, mucous membranes, and endothelia act as physical barriers, with tight cell junctions that block microbial passage.
Mechanical actions such as shedding of skin cells, mucociliary sweeping, coughing, peristalsis, and flushing (urination, tears) remove trapped microbes.
Resident microbiota compete with pathogens for binding sites and nutrients, inhibiting pathogen growth.

Chemical Defenses

Chemical mediators in body fluids and tissues provide nonspecific antimicrobial protection.

Enzymes in sebum, saliva, mucus, gastric fluids, urine, tears, cerumen, and vaginal secretions destroy pathogens.
Antimicrobial peptides (AMPs) such as dermcidin, cathelicidin, defensins, histatins, and bacteriocins are produced in response to pathogens.
Plasma proteins include acute-phase proteins, complement proteins, and cytokines.
Complement system is activated via classical, alternative, or lectin pathways, leading to opsonization, chemoattraction, inflammation, and cytolysis (membrane attack complex).
Cytokines regulate immune responses, including inflammation, cell proliferation, and differentiation.

Cellular Defenses

Blood contains formed elements that play key roles in innate immunity.

Red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes) originate from hematopoietic stem cells in bone marrow.
Granulocytes (neutrophils, eosinophils, basophils) are characterized by lobed nuclei and cytoplasmic granules.
Neutrophils fight bacterial infections; eosinophils target parasites; basophils and mast cells mediate allergic responses.
Natural killer (NK) cells destroy abnormal or infected cells via apoptosis.
Monocytes differentiate into macrophages and dendritic cells in tissues.

Pathogen Recognition and Phagocytosis

Phagocytes recognize and destroy pathogens through specialized receptors and processes.

Phagocytes use pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), to bind pathogen-associated molecular patterns (PAMPs).
Transendothelial migration allows leukocytes to move from blood to infected tissues.
Phagocytosis involves engulfing pathogens, digesting them in phagolysosomes, and excreting waste.

Inflammation and Fever

Inflammation and fever are systemic responses to infection or injury.

Acute inflammation is localized and short-lived; chronic inflammation can lead to granulomas and scarring.
Five cardinal signs: erythema, edema, heat, pain, altered function.
Fever raises body temperature and stimulates immune responses.
Excessive inflammation or fever can be harmful.

Adaptive Specific Host Defenses

Overview of Adaptive Immunity

Adaptive immunity is characterized by specificity and memory, providing targeted defense against pathogens.

Primary response: first exposure to antigen; secondary response: faster, stronger response upon re-exposure.
Humoral immunity (B cells, antibodies) vs. cellular immunity (T cells).
Antigens (immunogens) contain epitopes that elicit specific responses.
Antibodies (immunoglobulins) are Y-shaped glycoproteins with Fab (antigen-binding) and Fc (complement/opsonization) regions.
Five classes: IgM, IgG, IgA, IgE, IgD; functions include neutralization, opsonization, agglutination, complement activation, ADCC.

Major Histocompatibility Complexes (MHC) and Antigen-Presenting Cells (APCs)

MHC molecules are essential for antigen presentation and activation of adaptive immunity.

MHC I: expressed on all nucleated cells; presents self and foreign antigens for cytotoxic T cell recognition.
MHC II: expressed on APCs (macrophages, dendritic cells, B cells); presents processed antigens to helper T cells.
Antigen processing: APCs ingest pathogens, process antigens, and present immunodominant epitopes with MHC II.
Cross-presentation: dendritic cells present antigens on MHC I for T cell activation.

T Lymphocytes and Cellular Immunity

T cells mature in the thymus and are essential for cellular immunity.

Thymic selection: negative and positive selection ensures self-tolerance.
T cell receptor (TCR) diversity is generated by genetic rearrangement of V, D, J segments.
Classes: helper T cells (CD4), cytotoxic T cells (CD8), regulatory T cells.
Helper T cells differentiate into TH1, TH2, TH17, or memory T cells based on cytokine exposure.
Cytotoxic T cells kill infected cells via perforin and granzymes.
Superantigens cause nonspecific T cell activation and cytokine storm.

B Lymphocytes and Humoral Immunity

B cells produce antibodies and mediate humoral immunity.

B cells mature in bone marrow and spleen.
B cell receptors (BCRs) are membrane-bound IgD and IgM.
T-dependent antigens require helper T cell cooperation; T-independent antigens do not.
T-independent activation produces IgM only; T-dependent activation allows class switching and memory B cell formation.
Secondary antibody response is faster and stronger due to memory B cells.

Vaccines and Artificial Immunity

Vaccines stimulate artificial active immunity and promote herd immunity.

Types: natural active, natural passive, artificial active, artificial passive immunity.
Variolation (historical) vs. vaccination (modern).
Vaccine types: live attenuated, inactivated, subunit, toxoid, conjugate, RNA, vector vaccines.
Herd immunity requires a certain percentage of immune individuals to prevent disease spread.
R0 (basic reproduction number) is used to calculate herd immunity thresholds.

Formula for Herd Immunity Threshold:

Immune System Disorders

Hypersensitivities

Hypersensitivity reactions are exaggerated immune responses to antigens.

Type I: IgE-mediated, mast cell degranulation (allergies, anaphylaxis).
Type II: antibody-mediated cytotoxicity (hemolytic transfusion reaction, hemolytic disease of the newborn).
Type III: immune complex-mediated (tissue inflammation).
Type IV: T cell-mediated (delayed-type hypersensitivity).

Autoimmune Disorders

Autoimmune diseases result from loss of self-tolerance.

Organ-specific: celiac disease, Graves disease, Hashimoto thyroiditis, type I diabetes, Addison disease.
Systemic: multiple sclerosis, myasthenia gravis, psoriasis, rheumatoid arthritis, lupus.
Treatments: anti-inflammatory and immunosuppressive drugs.

Organ Transplantation and Rejection

Transplantation involves immune recognition of donor tissues.

Types: autograft, isograft, allograft, xenograft.
Human leukocyte antigens (HLAs) are critical for matching and avoiding rejection.
Immunosuppressive therapy is required to prevent rejection.
Graft-versus-host disease (GVHD) occurs when donor T cells attack recipient tissues.

Immunodeficiency

Immunodeficiencies are characterized by impaired immune function.

Primary: genetic defects (chronic granulomatous disease, X-linked agammaglobulinemia, selective IgA deficiency, SCID).
Secondary: acquired (malnutrition, viral infection, diabetes, prolonged infection, chemical/radiation exposure).
Treatments: depend on cause; may include immunotherapy or supportive care.

Cancer Immunobiology and Immunotherapy

The immune system can recognize and destroy cancer cells.

Cancer arises from uncontrolled cell proliferation and loss of differentiation.
Adaptive and innate responses target tumor antigens.
Immunotherapies include cytotoxic T cell stimulation and therapeutic vaccines.

Laboratory Analysis of the Immune Response

Polyclonal and Monoclonal Antibody Production

Antibodies are used in diagnostics and therapeutics.

Polyclonal antibodies: produced by immunizing animals; recognize multiple epitopes; used for screening.
Monoclonal antibodies (mAbs): produced from hybridomas; recognize single epitope; high specificity; used in diagnostics and cancer therapy.
Cross-reactivity is less problematic with mAbs.

Detecting Antigen-Antibody Complexes

Various assays detect antigen-antibody interactions.

Precipitin ring test, Ouchterlony assay, radial immunodiffusion visualize and quantify antigen-antibody lattices.
Flocculant formation is used in VDRL test for syphilis.
Virus-neutralizing antibodies are quantified in patient serum.
Immunoelectrophoresis identifies antibody classes.
Western blot detects specific antigens in serum.
Complement fixation test detects antibodies against pathogens.

Agglutination Assays

Agglutination tests are used for diagnosis and blood typing.

Direct agglutination: antibodies agglutinate cells/particles.
Indirect agglutination: latex beads detect antigens/antibodies.
Hemagglutination: used for blood typing and virus titration.
Coombs' test: detects antibodies bound to red blood cells.
Serotyping: identifies bacterial serovars.
Seroconversion is required for antibody detection.

EIAs and ELISAs

Enzyme immunoassays (EIA) and ELISAs are used to detect and quantify antigens or antibodies.

EIA: uses enzyme-conjugated antibodies; substrate conversion produces observable product.
Immunostaining: visualizes cells/tissues (immunohistochemistry, immunocytochemistry).
Direct ELISA: quantifies antigen; indirect ELISA: detects antibodies in serum.
Lateral flow tests: immunofiltration/immunochromatographic assays for rapid diagnosis (e.g., pregnancy, SARS-CoV-2).

Fluorescent Antibody Techniques

Immunofluorescence assays use fluorescent labels for rapid detection.

Direct immunofluorescence: detects antigens in clinical samples.
Indirect immunofluorescence: detects antigen-specific antibodies in patient sera.
Used for diagnosis of autoimmune diseases (antinuclear antibodies).
Flow cytometry: quantifies cell subsets using fluorescent mAbs.
Fluorescence-activated cell sorting (FACS): separates cells based on fluorescence intensity.

Additional Diagnostic Techniques

PCR and RT-PCR

Polymerase chain reaction (PCR) and reverse transcription PCR (RT-PCR) are nucleic acid amplification tests (NAATs) used in diagnostics.

PCR: amplifies DNA; used for pathogen detection.
RT-PCR: amplifies RNA by converting it to cDNA; used for RNA viruses (e.g., SARS-CoV-2).
SARS-CoV-2 RT-PCR Test: detects viral RNA in patient samples.
SARS-CoV-2 Rapid Antigen Test: detects viral proteins using lateral flow immunoassay.

Specimen Collection and Identification

Proper specimen collection is essential for accurate diagnosis.

Sample types: blood, fecal, urine, throat swab, sputum; selection depends on suspected infection.
Methods of identifying unknown microbes include serological and non-serological tests.
Lancefield classification: categorizes streptococci based on carbohydrate antigens.
Serotyping: identifies bacterial strains by antigenic properties.
Fluorescent in situ hybridization (FISH): uses fluorescent probes to detect specific nucleic acid sequences in cells.

Summary Table: Types of Immunity

Type	Source	Example
Natural Active	Exposure to pathogen	Recovery from chickenpox
Artificial Active	Vaccination	MMR vaccine
Natural Passive	Maternal antibodies	IgG transfer via placenta
Artificial Passive	Antibody injection	Antivenom for snakebite

Summary Table: Types of Vaccines

Type	Description	Example
Live Attenuated	Weakened pathogen	MMR, Varicella
Inactivated	Killed pathogen	Polio (IPV), Hepatitis A
Subunit	Antigenic fragments	Hepatitis B
Toxoid	Inactivated toxin	Diphtheria, Tetanus
RNA	mRNA encoding antigen	Moderna/Pfizer SARS-CoV-2
Vector	Viral vector delivers antigen gene	Johnson & Johnson SARS-CoV-2

Summary Table: Diagnostic Techniques

Technique	Principle	Application
ELISA	Enzyme-linked antibody detects antigen/antibody	HIV, SARS-CoV-2
PCR	Amplifies DNA	Pathogen detection
RT-PCR	Amplifies RNA via cDNA	RNA virus detection
Lateral Flow	Color-labeled antigen-antibody complex	Pregnancy, SARS-CoV-2 rapid test
Agglutination	Antibody-mediated clumping	Blood typing, pathogen detection
FISH	Fluorescent probe hybridization	Microbial identification

Additional info: The study guide covers all major aspects of innate and adaptive immunity, immune system disorders, and laboratory diagnostic techniques, including supplemental material on SARS-CoV-2 diagnostics and vaccines. The notes are expanded for academic completeness and exam preparation.