Immune System Disorders, Blood Groups, and Host Defense Mechanisms

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Immune System Disorders

Key Terms & Concepts

This section introduces important terms related to immune system disorders, hypersensitivity reactions, and autoimmune diseases.

Anaphylaxis: A severe, systemic allergic reaction that can cause hypotension, shock, nausea, vomiting, hives, and throat swelling. Immediate treatment is critical.
Autoimmune disorder: Occurs when the immune system mistakenly targets self-antigens, leading to tissue damage. Often involves type II hypersensitivity reactions.
IgE: An antibody class involved in allergic responses. Serum IgE levels may be measured during diagnosis, but elevated IgE alone does not confirm allergic disease.
Epinephrine: The first-line treatment for life-threatening anaphylaxis. It rapidly reverses airway constriction and cardiovascular collapse.
RhoGAM (human Rho(D) immune globulin): Administered to Rh-negative mothers during pregnancy and after delivery to prevent Rh incompatibility and hemolytic disease of the newborn.
Graves disease: An autoimmune disorder characterized by the production of TSH-receptor antibodies (thyroid-stimulating immunoglobulins), leading to hyperthyroidism.
Myasthenia gravis: Caused by antibodies targeting acetylcholine receptors at the neuromuscular junction, resulting in severe muscle weakness.

Blood Group System & Transfusions

ABO Blood Group Codominance

The ABO blood group system is determined by the presence of A and B antigens on red blood cells. The alleles exhibit codominance.

Codominance: Individuals with one A and one B allele express both antigens fully on their red blood cells.

Transfusion Reactions

Receiving the wrong blood type can trigger a dangerous immune response.

Symptoms: Hypotension, pruritus (itching), fever, chills, urticaria (hives), dyspnea (difficulty breathing), hemoglobinuria (hemoglobin in urine). Severe cases may result in shock and death.

Colitis

Definition and Pathogenesis

Colitis is inflammation of the colon, often due to disruption of the normal microbiota.

Cause: Overgrowth of nonresident bacteria, such as Clostridium difficile (a rod-shaped, gram-positive, obligate anaerobe).
Treatment: Fecal microbiota transplantation (FMT) can restore normal microbiota, outcompeting C. difficile.

Innate Immune Response to Wound Infection

Sequence of Events

The innate immune system provides immediate defense against pathogens entering through wounds.

Vasoconstriction: Occurs immediately after skin injury to reduce blood loss.
Mast cells release histamine: Increases blood flow and vascular permeability, allowing immune cells to access the site.
Leukocyte recruitment: Leukocytes exit blood vessels (diapedesis) and follow chemokines to the wound.
Phagocytosis and chemical attack: Leukocytes engulf pathogens or release antimicrobial chemicals.

Skin as a Barrier & Portal of Entry

Protective Functions of Skin

The skin acts as a primary barrier to infection, but can also serve as a portal of entry if breached.

Barrier properties: Keratin toughens the skin; dryness and acidity inhibit microbial growth; constant cell renewal removes attached microbes.
Portal of entry: Pathogens can enter through cuts or abrasions and spread to deeper tissues.

Microbiome as Innate Immune Barrier

Physical and Chemical Barriers

The normal microbiota provides protection against pathogens through competition and chemical defense.

Physical barrier: Competes with pathogens for colonization sites and nutrients.
Chemical barrier: Produces antimicrobial peptides (e.g., bacteriocins) that inhibit pathogen growth.

Interferons

Role in Antiviral Defense

Interferons are signaling proteins produced by virus-infected cells to limit viral spread.

Production: Synthesized by cells upon viral infection.
Effects: Induce nearby cells to reduce transcription and translation, inhibiting viral replication.
Immune activation: Stimulate immune cells to target and destroy virus-infected cells.

Antibody Functions

Five Major Functions

Antibodies (immunoglobulins) are critical for adaptive immunity and perform several key functions:

Neutralization: Bind to pathogens or toxins, preventing their interaction with host cells.
Opsonization: Coat pathogens to enhance phagocytosis by immune cells.
Agglutination: Clump pathogens together, facilitating their removal.
Complement activation: Trigger the complement cascade, leading to pathogen lysis.
Antibody-dependent cell-mediated cytotoxicity (ADCC): Direct immune cells to kill antibody-coated target cells.

Adaptive Immune System Characteristics

Specificity and Memory

The adaptive immune system is distinguished by its ability to target specific pathogens and remember previous encounters.

Specificity: Recognizes and targets specific antigens, minimizing damage to self-tissues.
Memory: Mounts a faster and stronger response upon re-exposure to the same pathogen.

Additional Definitions

Opportunistic pathogens: Microbes that cause disease primarily when host immunity is compromised.
Virulence factors: Molecules produced by pathogens that enable them to cause disease (e.g., toxins, adhesins).
Portals of entry: Anatomical sites where pathogens can enter host tissue (e.g., skin, mucous membranes).
Toxin: A biological poison produced by pathogens that can damage host tissues.

Influenza Vaccine

Antigenic Variation and Vaccine Updates

The influenza virus undergoes frequent genetic changes, necessitating annual vaccine updates.

Antigenic drift: Gradual accumulation of point mutations in viral genes, altering surface proteins.
Antigenic shift: Sudden gene reassortment, creating new viral strains with novel antigens.
Vaccine implication: Annual vaccination is required to match circulating strains.

Table: Summary of Key Immune Disorders and Mechanisms

Disorder/Concept	Mechanism	Key Features
Anaphylaxis	Systemic allergic reaction (Type I hypersensitivity)	Hypotension, shock, airway swelling; treated with epinephrine
Autoimmune disorder	Immune response against self-antigens (Type II hypersensitivity)	Tissue-specific damage (e.g., Graves disease, myasthenia gravis)
Colitis (C. difficile)	Disruption of normal microbiota; overgrowth of pathogen	Inflammation, treated with fecal transplant
Blood transfusion reaction	Antibody-mediated destruction of transfused RBCs	Fever, chills, shock, hemoglobinuria
Influenza vaccine	Induces immunity to viral antigens	Updated yearly due to antigenic drift/shift