1.5 Immune System, Disease & Homeostasis

1.5 Immune System, Disease & Homeostasis

To survive in an environment filled with microscopic pathogens, living organisms must defend their internal body integrity and maintain stable internal physical and chemical conditions. On the GED Science test, questions evaluate your knowledge of homeostatic feedback loops, pathogen classifications, innate vs. adaptive immune responses, and the biological mechanisms of vaccines and antibiotics.

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Understanding Homeostasis and Dynamic Equilibrium

Homeostasis is the ability of an organism or cell to maintain relatively stable internal conditions despite continuous changes in the external environment. Parameters regulated homeostatically include:

• Body temperature (approx. $37^\circ\text{C}$ or $98.6^\circ\text{F}$ in humans)
• Blood glucose concentration (approx. $70\text{--}100\text{ mg/dL}$ fasting)
• Blood pH (strictly maintained between $7.35\text{--}7.45$)
• Water balance and dissolved electrolyte concentrations

Homeostasis does not mean internal conditions are static; rather, it is a dynamic equilibrium where conditions fluctuate within a narrow, healthy tolerance range around a set point.

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Negative Feedback vs. Positive Feedback Loops

The body regulates homeostatic parameters through control loops featuring three core components:

1. Sensor (Receptor): Detects deviations from the normal set point.
2. Control Center (Integration Center): Processes sensory data and sends signals to effectors (e.g., hypothalamus in brain).
3. Effector: Organ or tissue that responds to alter the internal condition.

Negative Feedback Loops (Primary Regulator)

In a negative feedback loop, the system's output reverses or counteracts the initial stimulus, driving the variable back toward the normal set point.

• Example 1: Thermoregulation (Body Temperature Control)
  • Stimulus: Body temperature rises above $37^\circ\text{C}$ (e.g., during exercise).
  • Sensors: Thermoreceptors in skin and hypothalamus detect heat.
  • Effectors: Sweat glands secrete sweat (evaporative cooling); blood vessels in skin dilate (vasodilation) to radiate heat $\rightarrow$ Temperature drops back to $37^\circ\text{C}$.
  • Stimulus: Body temperature drops below $37^\circ\text{C}$.
  • Effectors: Skeletal muscles shiver (generate heat); skin blood vessels constrict (vasoconstriction) $\rightarrow$ Temperature rises back to $37^\circ\text{C}$.
• Example 2: Blood Glucose Regulation
  • High Blood Glucose (Post-Meal): Pancreas releases insulin $\rightarrow$ Cells absorb glucose and liver converts glucose into stored glycogen $\rightarrow$ Blood sugar lowers.
  • Low Blood Glucose (Fasting): Pancreas releases glucagon $\rightarrow$ Liver breaks down stored glycogen into free glucose $\rightarrow$ Blood sugar rises.

Positive Feedback Loops (Amplifying Response)

In a positive feedback loop, the output amplifies or reinforces the initial stimulus, moving the system further away from the set point until a definitive endpoint is achieved.

• Example: Childbirth (Parturition) $\rightarrow$ Baby pushes against cervix $\rightarrow$ Oxytocin hormone released $\rightarrow$ Uterine contractions intensify $\rightarrow$ Pushes baby further $\rightarrow$ More oxytocin released until delivery.

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Pathology: Infectious vs. Non-Infectious Diseases

A disease is any impairment of normal physiological body function. Diseases are classified into two major categories:

1. Infectious Diseases: Caused by microscopic invading organisms (pathogens) that can be transmitted between hosts.
2. Non-Infectious Diseases: Caused by genetic mutations, environmental toxin exposure, nutritional deficiencies, or lifestyle factors (e.g., diabetes, heart disease, cancer).

Major Pathogen Categories

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The Immune System: First and Second Lines of Defense (Innate)

The human immune system is organized into three defense lines. The first two lines constitute Innate (Non-Specific) Immunity, which responds immediately to any invader regardless of identity.

First Line of Defense: Physical and Chemical Barriers

• Skin: Intact keratinized epidermis forms an impermeable physical barrier.
• Mucous Membranes: Sticky mucus traps inhaled pathogens in respiratory tracts; cilia sweep mucus up and out.
• Chemical Secretions: Lysozyme enzymes in tears/saliva destroy bacterial cell walls; gastric acid ($HCl$) destroys ingested microbes.

Second Line of Defense: Cellular & Chemical Innate Defenses

• Phagocytes: Specialized white blood cells (macrophages and neutrophils) that engulf and digest foreign pathogens via phagocytosis.
• Inflammatory Response: Damaged cells release histamine, causing local vasodilation (increased blood flow $\rightarrow$ redness and heat) and capillary permeability (fluid leakage $\rightarrow$ swelling/edema). Brings white blood cells rapidly to infection site.
• Fever: Pyrogen chemicals signal the hypothalamus to raise body temperature, inhibiting pathogen replication and accelerating white blood cell activity.

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Adaptive Immunity: B-Cells, T-Cells, Antibodies, and Memory

If pathogens breach innate barriers, the Third Line of Defense: Adaptive (Acquired) Immunity activates. Adaptive immunity is antigen-specific, targeted, and creates long-lasting immunological memory.

An antigen is any foreign molecular surface marker (usually a protein or glycoprotein on a pathogen) that triggers an immune response.

Two Branches of Adaptive Immunity

1. Humoral Immunity (B-Cell Mediated):

   • B-Lymphocytes (B-Cells) mature in bone marrow.
   • When a B-cell encounters its specific matching antigen, it multiplies and differentiates into:
     • Plasma Cells: Secrete massive quantities of specialized $Y$-shaped proteins called antibodies into blood plasma.
     • Memory B-Cells: Persist in lymph nodes for decades.
   • Antibody Function: Antibodies bind specifically to pathogen antigens, neutralizing viruses, clumping pathogens together (agglutination), and tagging them for destruction by phagocytes.

2. Cell-Mediated Immunity (T-Cell Mediated):

   • T-Lymphocytes (T-Cells) mature in the thymus gland.
   • Helper T-Cells ($CD4^+$): Release chemical signals (cytokines) to coordinate both B-cell and Cytotoxic T-cell activation.
   • Cytotoxic T-Cells ($CD8^+$): Directly target and destroy virus-infected host cells and cancer cells by secreting pore-forming proteins (perforins).

Primary vs. Secondary Immune Response

• Primary Response: First exposure to a novel pathogen. Takes 7-14 days to build effective antibody levels; individual experiences disease symptoms.
• Secondary Response: Subsequent exposure to the same pathogen. Memory B and T cells recognize the antigen instantly, producing vast antibody quantities within hours. Pathogen is eliminated before symptoms develop (immunity).

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Active vs. Passive Immunity and Vaccine Science

How Vaccines Work

Vaccines contain weakened (attenuated), inactivated, or fragment pieces of a pathogen (antigens). The vaccine safely stimulates the primary adaptive immune response—producing antibodies and memory cells—without causing the disease. When exposed to the real live pathogen later, the body executes a swift secondary immune response.

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Antibiotic Action and the Crisis of Antibiotic Resistance

Antibiotics are biochemical drugs that selectively kill or inhibit bacteria. They exploit structural differences between prokaryotic bacterial cells and eukaryotic human cells:

• Blocking bacterial cell wall synthesis (e.g., penicillin targeting peptidoglycan).
• Inhibiting prokaryotic 70S ribosome protein synthesis (e.g., tetracycline).

Critical GED Distinction: Antibiotics DO NOT kill viruses. Viruses lack cell walls, ribosomes, and independent metabolic machinery. Prescribing antibiotics for viral infections like flu or common colds is ineffective and accelerates antibiotic resistance—where mutant bacteria with resistance genes survive, reproduce, and spread.

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Exam Strategy & Worked Example

GED-Style Data Analysis Problem

Scenario: Graph plotting blood antibody concentration over time in a patient exposed to Antigen X on Day 0 and exposed to Antigen X again on Day 40:

Question: Based on the data, how does the response to the second exposure on Day 40 compare to the first exposure on Day 0, and what cellular mechanism explains this difference?

Step-by-Step Solution:

1. Compare Response Lag Time: Following initial exposure (Day 0), antibody production took 7 days to start and 14 days to peak. Following second exposure (Day 40), antibody production peaked dramatically in just 3 days (Day 43).
2. Compare Peak Antibody Levels: Peak concentration after second exposure ($550\ \mu\text{g/mL}$) was over 20 times higher than after initial exposure ($25\ \mu\text{g/mL}$).
3. Identify Cellular Mechanism: The initial exposure generated long-lived Memory B-cells and Memory T-cells. Upon second exposure, these memory cells recognized Antigen X immediately, rapidly proliferating into plasma cells that secreted vast antibody volumes.