Chapter 19: Immunological Disorders and HIV Infection

Immunological Disorders

Hygiene Hypothesis

The Hygiene Hypothesis proposes that reduced exposure to microbes during early childhood may impair the development of immune tolerance. This can result in the immune system overreacting to harmless antigens, leading to increased rates of allergies and autoimmune diseases.

• Key Point: Early microbial exposure is important for proper immune system development.

• Example: Children raised in overly sterile environments may have higher rates of allergies.

Conditions Linked to Microbial Imbalances

Imbalances or limited exposure to microbes are associated with several health conditions:

• Allergies

• Asthma

• Inflammatory Bowel Diseases (e.g., Crohn's disease, Ulcerative colitis)

• Dysbiosis: Microbial imbalance, often caused by antibiotic use

Mechanism of Autoimmune Diseases

Autoimmune diseases occur when the immune system loses self-tolerance and attacks the body's own cells (self-antigens), resulting in tissue and organ damage.

• Key Point: Loss of self-tolerance leads to immune-mediated destruction of healthy tissues.

• Example: Type 1 diabetes, where immune cells attack pancreatic beta cells.

Immunodeficiencies: Congenital vs. Acquired

Immunodeficiencies are classified based on their origin:

• Congenital Immunodeficiencies: Present at birth, caused by defective or missing genes. Example: DiGeorge syndrome.

• Acquired Immunodeficiencies: Develop later in life, caused by infections, drugs, or cancers. Example: AIDS resulting from HIV infection.

HIV Infection and AIDS

HIV Evasion of Host Antibodies

Human Immunodeficiency Virus (HIV) employs several strategies to evade the host's immune response:

• Remains latent as a provirus integrated into host cell DNA

• Rapid antigenic mutation alters viral surface proteins

• Direct cell-to-cell transmission via fusion

Origin of HIV in Humans

HIV originated from Simian Immunodeficiency Virus (SIV) in chimpanzees and crossed into humans in West/Central Africa around 1908 through contact with infected bushmeat.

• Key Point: Zoonotic transmission from primates to humans.

Stages of HIV Infection

HIV infection progresses through three main phases:

• Phase 1 – Acute Infection (Asymptomatic):

  • High viral load

  • CD4 T cells decrease, then partially recover

• Phase 2 – Clinical Latency (Symptomatic):

  • HIV continues multiplying

  • CD4 T cells gradually decline

• Phase 3 – AIDS Indicator Conditions:

  • CD4 count drops below 200 cells/μL

  • Severe immune system damage and opportunistic infections

Effects of HIV on the Immune System

HIV infects and destroys CD4+ helper T cells, weakening the immune system and increasing susceptibility to opportunistic infections and cancers.

• Key Point: Loss of CD4+ T cells impairs both innate and adaptive immunity.

Routes of HIV Transmission

HIV can be transmitted through several routes:

• Sexual contact

• Blood transfusions

• Contaminated needles

• Organ transplants

• Artificial insemination

• Breast milk

• Transplacental transmission (mother to fetus)

Prevention and Treatment of HIV Infection

Current methods for preventing and treating HIV infection include:

• Prevention:

  • Condom use

  • HIV testing and counseling

  • Needle-exchange programs

  • Safe infant feeding practices

• Treatment:

  • HAART (Highly Active Antiretroviral Therapy)

  • PrEP (Pre-Exposure Prophylaxis)

  • PEP (Post-Exposure Prophylaxis)

Summary Table: HIV Infection Stages and Immune Effects

Stage	Viral Load	CD4 T Cell Count	Symptoms
Phase 1: Acute Infection	High	Decreases, then partial recovery	Usually asymptomatic
Phase 2: Clinical Latency	Moderate	Gradual decline	Symptomatic
Phase 3: AIDS	High	< 200 cells/μL	Severe immune damage, opportunistic infections

Key Definitions

• Self-tolerance: The immune system's ability to avoid attacking the body's own cells.

• Dysbiosis: An imbalance in the microbial populations of the body, often linked to disease.

• Opportunistic infection: Infections caused by organisms that take advantage of weakened immune systems.

• HAART: Combination therapy using multiple antiretroviral drugs to suppress HIV replication.

*Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.*