3D Microbio

Hypersensitivity (Allergic Reaction)

An exaggerated or overreaction of the immune system to an antigen; there are four types of hypersensitivity reactions.

Type 1 Hypersensitivity

Immediate allergy involving IgE antibodies, mast cells, and rapid release of inflammatory mediators after exposure to an allergen.

Atopy

Any chronic local allergy such as hay fever or asthma.

Anaphylaxis

A systemic, sometimes fatal allergic reaction involving airway obstruction and circulatory collapse.

Allergen

An antigen that triggers a Type 1 hypersensitivity (allergic) response.

Four Classifications of Allergens

Inhalants, Ingestants, Injectants, and Contactants.

Inhalant Allergens

Allergens that enter through inhalation; examples include pollen, dust, mold spores, and drugs.

Ingestant Allergens

Allergens that enter through ingestion; examples include food, drugs, and food additives.

Injectant Allergens

Allergens introduced by injection; examples include bee/wasp venom, drugs, and vaccines.

Contactant Allergens

Allergens that trigger reactions through skin contact; examples include drugs, cosmetics, detergents, dyes, and heavy metals.

Sensitization Dose

The initial encounter with an allergen that stimulates the immune response and production of IgE antibodies.

Antibody Produced During Type 1 Hypersensitivity

IgE.

IgE

Antibody produced by plasma cells during sensitization that binds to mast cells and primes them for future allergen exposure.

Mast Cells

Cells that bind IgE antibodies and contain chemical mediators that are released during allergic reactions.

Degranulation

Release of chemical mediators from mast cells after IgE binds an allergen during subsequent exposures.

Histamine

Fastest and most abundant allergic mediator; constricts smooth muscles of bronchioles and intestines, relaxes vascular smooth muscle, and causes wheal and flare reactions, itching (pruritus), and headache.

Serotonin

Allergic mediator similar to histamine but with reduced central nervous system activity.

Leukotriene

Slow-reacting allergic mediator responsible for prolonged bronchospasm, increased vascular permeability, and mucus secretion in asthma.

Prostaglandins

Powerful inflammatory mediators that also stimulate uterine contractions during childbirth.

Bradykinin

Allergic mediator related to the kinin family of peptides.

Examples of Atopic Diseases

Hay fever, allergic asthma, eczema, food allergy, and drug allergy.

Symptoms of Food Allergy

A variety of respiratory and gastrointestinal symptoms.

Cutaneous Anaphylaxis

Local wheal and flare inflammatory reaction at the site where an allergen is injected.

Systemic Anaphylaxis (Anaphylactic Shock)

Sudden respiratory and circulatory disruption that may be fatal within minutes; example: severe bee sting reaction.

Desensitization Therapy

Allergy treatment involving repeated small doses of an allergen to retrain the immune system to reduce its overreaction.

Type 2 Hypersensitivity

Complement-assisted destruction of cells by IgG and IgM antibodies directed against cell surface antigens.

Antibodies Involved in Type 2 Hypersensitivity

IgG and IgM.

Universal Donor Blood Type

Type O because it lacks A and B antigens.

Universal Recipient Blood Type

Type AB because it lacks anti-A and anti-B antibodies.

Effects of Incompatible Blood Transfusion

Antigen-antibody complexes activate complement, causing hemolysis, fever, anemia, systemic shock, kidney failure, and possibly death.

Rh Factor (D Antigen)

Blood antigen important because it can cause Hemolytic Disease of the Newborn.

Hemolytic Disease of the Newborn (HDN)

Disease caused when maternal antibodies attack Rh-positive fetal red blood cells.

How Rh- Mothers Develop Anti-Rh Antibodies

Through placental sensitization during pregnancy or an incorrect blood transfusion.

RhoGAM

Anti-Rh antibody injection given to Rh-negative mothers to prevent maternal antibodies from attacking fetal Rh-positive cells.

Condition Required for HDN

Rh-positive fetal blood enters an Rh-negative mother, allowing sensitization.

Type 3 Hypersensitivity

Immune complex reaction where soluble antigen-antibody complexes deposit in basement membranes and activate complement.

Similarity Between Type 2 and Type 3 Hypersensitivity

Both involve IgG and IgM antibodies after repeated antigen exposure and activate complement.

Difference Between Type 2 and Type 3 Hypersensitivity

Type 2 antigens are attached to cell surfaces, while Type 3 antigens are soluble and not attached to cells.

Arthus Reaction

Acute Type 3 hypersensitivity response after a second vaccine injection (booster); usually self-limiting but may occasionally cause blood clotting and tissue damage.

Serum Sickness

Type 3 hypersensitivity caused by repeated exposure to horse serum, animal hormones, or certain drugs.

Type 4 Hypersensitivity

Delayed-type hypersensitivity mediated by T cells; symptoms appear one to several days after second antigen exposure.

Cells Involved in Type 4 Hypersensitivity

T cells.

Targets of Type 4 T Cells

Self tissues or transplanted foreign cells displaying antigens.

Examples of Type 4 Hypersensitivity

Delayed allergic reactions to infectious agents, contact dermatitis, and graft rejection.

Tuberculin (TB) Skin Test

Example of a Type 4 infectious allergy.

Positive TB Skin Test (Without Vaccination)

Indicates previous infection with Mycobacterium tuberculosis.

TB Skin Test Reaction Time

24-48 hours.

Difference Between Type 4 and Types 1-3

Type 4 is T-cell mediated, while Types 1, 2, and 3 primarily involve B cells and antibodies.

Immunodeficiency

Failure of the immune system to function properly; classified as primary or secondary.

Primary Immunodeficiency

Congenital immunodeficiency present at birth, usually caused by genetic defects; example: Severe Combined Immunodeficiency (SCID).

Secondary Immunodeficiency

Acquired after birth due to natural or artificial causes; example: AIDS.

SCID

Severe Combined Immunodeficiency; inherited disease causing a severely weakened immune system.

AIDS

Acquired Immunodeficiency Syndrome; secondary immunodeficiency characterized by depletion of helper T cells.

Importance of Proper Specimen Collection

Ensures accurate and reliable culture results; poor specimens produce poor results ("garbage in, garbage out").

Reasons Proper Specimen Collection Is Important

Prevents contamination, uses aseptic technique, requires sterile containers, proper transport media, and timely delivery because organisms may change over time.

Serology

Branch of immunology that studies antigen-antibody interactions in serum using in vitro diagnostic testing.

Specificity

Ability of a test to react only with the intended antigen or antibody and not unrelated ones.

Sensitivity

Ability of a test to detect even very small amounts of the target antigen or antibody.

Known Antibody Used to Detect

An unknown antigen.

Known Antigen Used to Detect

An unknown antibody.

Agglutination

Visible antigen-antibody reaction involving whole cells or particles.

Precipitation

Visible antigen-antibody reaction involving soluble (cell-free) antigens and antibodies.

Titer

Quantity of antibodies in serum determined by serial dilution; equals the highest dilution that still produces a visible positive reaction.

How a Titer Is Determined

By serially diluting serum with antigen until the highest dilution that still shows agglutination is found.

How to Report a Titer

Report the reciprocal of the highest positive dilution.

High Titer

Indicates a greater concentration of antibodies in circulation and greater protection against the antigen.

Low Titer

Indicates fewer antibodies in circulation and less protection.

Patient with More Antibodies (Titer 160 vs. 40)

Patient A (160) because the sample required greater dilution before antibodies were no longer detectable.

Patient with Greater Protection (Titer 160 vs. 40)

Patient A because higher antibody levels provide greater protection.

Example Titer Calculation

If 1/80 is the highest dilution with a positive reaction and 1/160 is negative, the titer is 1/80 (reported as reciprocal value 80 if requested).