NU551 Unit 2 Study Guide: Immunity, Infection, and Stress Overview

NU551 Unit 2 Study Guide: Immunity, Infection, and Stress Overview

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Purdue University Global

NU551 Advanced Physiology and Pathophysiology Across the Lifespan

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NU551 Unit 2 Study Guide: Immunity, Infection, and Stress Overview

The immune system protects the body through innate immunity and adaptive immunity, which work together to prevent infection and promote recovery. Innate immunity provides immediate, non-specific protection, while adaptive immunity creates targeted, long-term defense through immune memory. When pathogens invade the body, inflammation helps eliminate harmful microorganisms, but chronic stress can suppress immune function by increasing cortisol levels, making infections more likely and slowing healing.

Understanding how immunity, infection, inflammation, hypersensitivity reactions, and stress interact is essential for nursing students preparing for exams and clinical practice.

Understanding Innate Immunity

Innate immunity is the body’s first line of defense against infectious organisms. It responds within minutes of exposure and does not require previous contact with a pathogen. Although it lacks specificity, innate immunity prevents many infections from becoming established.

The innate immune system protects the body through several defense mechanisms.

• Physical barriers: Skin and mucous membranes prevent pathogens from entering the body.

• Chemical barriers: Gastric acid, lysozyme in tears and saliva, and antimicrobial secretions destroy microorganisms before they spread.

• Cellular defenses: Neutrophils, macrophages, dendritic cells, and natural killer (NK) cells identify, engulf, and eliminate invading pathogens.

These defenses also initiate inflammation, which recruits additional immune cells to the affected tissues and begins the healing process.

Understanding Adaptive Immunity

Adaptive immunity develops after exposure to pathogens or vaccines and provides highly specific immune protection. Unlike innate immunity, it remembers previous infections, allowing the immune system to respond faster during future exposures.

The adaptive immune system relies primarily on specialized lymphocytes.

B Lymphocytes (B Cells)

B cells mature into plasma cells that produce antibodies against specific antigens. These antibodies neutralize bacteria, viruses, and toxins while marking pathogens for destruction.

T Lymphocytes (T Cells)

T cells play different roles in coordinating immune responses.

• CD4+ Helper T Cells: Activate and regulate other immune cells.

• CD8+ Cytotoxic T Cells: Destroy virus-infected, cancerous, and abnormal cells.

Memory Cells

Following infection or vaccination, memory B and T cells remain in the body. Their presence enables a quicker and stronger immune response during future encounters with the same pathogen, forming the biological foundation of vaccine-induced immunity.

Innate vs. Adaptive Immunity

How Inflammation Supports Immune Defense

Inflammation is a protective response that occurs when tissues are injured or infected. Its primary purpose is to eliminate harmful microorganisms, remove damaged tissue, and promote healing.

Typical signs of inflammation include:

• Fever

• Redness

• Warmth

• Swelling

• Pain

• Fatigue

• Leukocytosis (elevated white blood cell count)

• Increased C-reactive protein (CRP)

Although inflammation is essential for recovery, excessive or prolonged inflammation may contribute to tissue damage and chronic disease.

Common Infectious Agents

Infections develop when microorganisms invade the body, multiply, and overcome host defense mechanisms.

The major categories of infectious organisms include:

• Bacteria

• Viruses

• Fungi

• Parasites

Once infection occurs, the innate immune response activates first, followed by the adaptive immune response if the pathogen persists.

Types of Hypersensitivity Reactions

Hypersensitivity reactions occur when the immune system responds excessively or inappropriately to harmless substances or self-antigens. These reactions are classified into four major types.

Type I: Immediate (IgE-Mediated) Hypersensitivity

Type I reactions occur within minutes after allergen exposure. Immunoglobulin E (IgE) triggers mast cells to release histamine and other inflammatory mediators.

Common clinical examples include:

• Anaphylaxis

• Allergic rhinitis

• Asthma

• Food allergies

Type II: Cytotoxic Hypersensitivity

Type II reactions occur when IgG or IgM antibodies attack cells carrying specific antigens, leading to cell destruction.

Examples include:

• Hemolytic anemia

• Blood transfusion reactions

• Hemolytic disease of the newborn

Type III: Immune Complex Hypersensitivity

Type III reactions develop when antigen-antibody complexes accumulate in tissues and blood vessels, triggering inflammation and tissue injury.

A common example is:

• Systemic lupus erythematosus (SLE)

Type IV: Delayed (Cell-Mediated) Hypersensitivity

Type IV hypersensitivity is mediated by T lymphocytes rather than antibodies. Symptoms typically develop 24 to 72 hours after exposure.

Common examples include:

• Tuberculin (TB) skin test

• Contact dermatitis

• Poison ivy reactions

How Stress Affects the Immune System

Stress influences immune function through activation of the hypothalamic-pituitary-adrenal (HPA) axis. During stressful situations, the body releases hormones that help maintain homeostasis. However, prolonged activation suppresses immune defenses.

The stress response follows this sequence:

1. The hypothalamus detects stress.

2. The pituitary gland releases adrenocorticotropic hormone (ACTH).

3. The adrenal glands produce cortisol.

4. Cortisol increases blood glucose while suppressing immune cell activity.

Acute stress can temporarily support survival, but chronic elevation of cortisol weakens immune function.

Health Effects of Chronic Stress

Persistent stress contributes to immune dysregulation and increases vulnerability to both infectious and chronic illnesses.

Long-term health consequences include:

• Reduced immune response

• Delayed wound healing

• Increased susceptibility to infections

• Depression and anxiety

• Cardiovascular disease

• Elevated blood glucose levels

• Chronic inflammation

Healthy lifestyle habits—including regular exercise, adequate sleep, balanced nutrition, relaxation techniques, and stress management—help strengthen immune resilience and improve overall health.

Key Differences Between Acute and Chronic Stress

Key Points for Nursing Practice

Nursing professionals should recognize that effective immune function depends on the interaction between innate and adaptive immunity. Understanding immune responses helps clinicians identify infections, interpret inflammatory markers, recognize hypersensitivity reactions, and educate patients about disease prevention.

Stress assessment should also be incorporated into patient care because prolonged stress negatively affects immune competence and recovery outcomes.

Quick Review

• Innate immunity provides immediate, non-specific protection.

• Adaptive immunity develops antigen-specific responses and long-term immune memory.

• B cells produce antibodies, while T cells regulate and eliminate infected cells.

• Inflammation is a protective response that supports infection control and tissue repair.

• Four hypersensitivity reactions differ by immune mechanism, timing, and clinical presentation.

• Chronic stress activates the HPA axis, increases cortisol production, and suppresses immune function.

Key Facts About Immunity, Infection, and Stress

• Innate immunity is the body’s first defense against pathogens and responds immediately without prior exposure.

• Adaptive immunity develops after exposure to specific antigens and provides lasting protection through memory cells.

• B lymphocytes produce antibodies, while CD4+ and CD8+ T cells coordinate and execute cellular immune responses.

• Type I through Type IV hypersensitivity reactions differ in immune mechanisms, timing, and clinical manifestations.

• Chronic stress elevates cortisol levels, suppresses immune cell activity, delays healing, and increases susceptibility to infection.

Frequently Asked Questions

What is the difference between innate and adaptive immunity?

Innate immunity provides immediate, non-specific protection using physical barriers and immune cells. Adaptive immunity develops a targeted response through B and T lymphocytes and creates long-lasting immune memory.

Which immune cells produce antibodies?

B lymphocytes differentiate into plasma cells, which produce antibodies that recognize and neutralize specific pathogens.

What are CD4+ and CD8+ T cells?

CD4+ helper T cells coordinate immune responses by activating other immune cells, whereas CD8+ cytotoxic T cells directly destroy virus-infected and abnormal cells.

What are the four types of hypersensitivity reactions?

The four types include:

• Type I: Immediate, IgE-mediated allergic reactions

• Type II: Cytotoxic reactions mediated by IgG or IgM

• Type III: Immune complex-mediated inflammation

• Type IV: Delayed, T-cell-mediated reactions

What causes inflammation during infection?

Inflammation occurs when immune cells release chemical mediators in response to pathogens or tissue injury. This process recruits additional immune cells, removes harmful microorganisms, and initiates tissue repair.

How does chronic stress weaken immunity?

Chronic stress activates the HPA axis, leading to prolonged cortisol release. Persistently elevated cortisol suppresses immune cell function, delays wound healing, increases infection risk, and contributes to chronic diseases.

Why is adaptive immunity important for vaccination?

Vaccines stimulate adaptive immunity by generating memory B and T cells. These memory cells enable the immune system to recognize and rapidly eliminate pathogens during future exposures.

References

Abbas, A. K., Lichtman, A. H., & Pillai, S. (2023). Cellular and molecular immunology (10th ed.). Elsevier. https://www.elsevier.com/books/cellular-and-molecular-immunology/abbas/978-0-323-75748-5

Kumar, V., Abbas, A. K., & Aster, J. C. (2024). Robbins & Cotran pathologic basis of disease (11th ed.). Elsevier. https://www.elsevier.com/books/robbins-and-cotran-pathologic-basis-of-disease/kumar/978-0-323-53113-9

McCance, K. L., Huether, S. E., Brashers, V. L., & Rote, N. S. (2023). Pathophysiology: The biologic basis for disease in adults and children (9th ed.). Elsevier. https://www.elsevier.com/books/pathophysiology-the-biologic-basis-for-disease-in-adults-and-children/mccance/978-0-323-78305-7

NU551 Unit 2 Study Guide: Immunity, Infection, and Stress Overview

OpenStax. (2023). Anatomy and Physiology 2e: The immune system. https://openstax.org/books/anatomy-and-physiology-2e/pages/21-introduction

World Health Organization. (2024). Stress. https://www.who.int/news-room/questions-and-answers/item/stress