The Lines of Defense

The human body has multiple layers of defense against pathogens and injury, broadly categorized into three lines:

1. First Line of Defense: Non-specific barriers that prevent entry of pathogens.

2. Second Line of Defense: Non-specific internal defenses activated if pathogens breach the first line.

3. Third Line of Defense: Specific, adaptive immunity that targets particular pathogens.

First Line of Defense: Physical and Chemical Barriers

These are the body’s initial, non-specific defenses, acting as physical or chemical deterrents to pathogen invasion. In surgical nursing, maintaining the integrity of these barriers is crucial.

• Skin: The largest organ, providing a tough, impermeable physical barrier. Intact skin is vital in preventing surgical site infections. Surgical incisions breach this barrier, highlighting the importance of sterile technique.

  • Relevance: Pre-operative skin preparation, sterile draping, wound care.

• Mucous Membranes: Line body cavities (respiratory, gastrointestinal, genitourinary tracts) and secrete mucus, trapping pathogens. Cilia in the respiratory tract sweep mucus and trapped particles away.

  • Relevance: Maintaining respiratory hygiene, preventing urinary tract infections (UTIs) in catheterized patients, managing nasogastric tubes.

• Secretions: Various bodily fluids contain antimicrobial substances.

  • Tears and Saliva: Contain lysozyme, an enzyme that breaks down bacterial cell walls.

  • Gastric Acid: Low pH in the stomach kills many ingested pathogens.

  • Urine: Acidic pH and flushing action help prevent UTIs.

  • Sebum: Fatty acids on the skin inhibit bacterial growth.

  • Relevance: Oral hygiene, managing feeding tubes, monitoring urine output and characteristics.

• Normal Flora: Commensal microorganisms on body surfaces compete with pathogens for nutrients and space, and can produce antimicrobial substances.

  • Relevance: Understanding the risk of opportunistic infections when normal flora is disrupted (e.g., by antibiotics).

Second Line of Defense: Innate Immunity

If pathogens bypass the first line, the innate immune system provides a rapid, non-specific response. This system is always ready to act.

• Inflammation: A localized tissue response to injury or infection, characterized by redness, swelling, heat, pain, and loss of function. Its purpose is to:

  • Prevent spread of the damaging agent.

  • Dispose of cell debris and pathogens.

  • Set the stage for repair.

  • Key Players: Mast cells (release histamine), phagocytes (neutrophils, macrophages), inflammatory mediators (cytokines, prostaglandins).

  • Relevance: Recognizing signs of surgical wound infection, understanding post-operative swelling and pain as part of the healing process, monitoring inflammatory markers (e.g., C-reactive protein).

• Phagocytosis: Process by which phagocytic cells (neutrophils, macrophages) engulf and destroy pathogens and cellular debris.

  • Relevance: Assessing white blood cell counts (especially neutrophils) as indicators of infection, understanding how the body clears foreign material from the surgical site.

• Fever: A systemic increase in body temperature, often triggered by pyrogens released by pathogens or immune cells. Moderate fever can:

  • Inhibit growth of some microorganisms.

  • Increase metabolic rate, speeding up repair processes.

  • Enhance immune cell activity.

  • Relevance: Monitoring patient temperature post-operatively, recognizing fever as a potential sign of infection, implementing fever management strategies.

• Natural Killer (NK) Cells: Lymphocytes that kill infected body cells and cancer cells by releasing cytotoxic chemicals.

  • Relevance: Contributing to the body’s defense against viral infections and malignancy, which can be concerns in surgical patients.

• Antimicrobial Substances:

  • Complement System: A group of plasma proteins that, when activated, can lyse pathogens, enhance phagocytosis (opsonization), and amplify inflammation.

  • Interferons: Proteins released by virus-infected cells that signal nearby cells to produce antiviral proteins, inhibiting viral replication.

  • Relevance: Understanding how these systems contribute to fighting infections, especially in immunocompromised patients.

Third Line of Defense: Adaptive (Acquired) Immunity

This is a specific, slower response that targets particular pathogens and develops a memory of them for future encounters. While primarily the domain of immunology, surgical nurses should have a basic understanding of its components as it relates to long-term immunity and vaccination.

• Lymphocytes:

  • B Cells: Produce antibodies (immunoglobulins) that target specific antigens on pathogens.

  • T Cells: Include Helper T cells (coordinate the immune response) and Cytotoxic T cells (directly kill infected cells).

• Antigen-Presenting Cells (APCs): Such as macrophages and dendritic cells, which present antigens to T cells to initiate the adaptive response.

• Memory Cells: B and T cells that “remember” a specific pathogen, allowing for a faster and stronger response upon re-exposure.

  • Relevance: Understanding the importance of vaccination (building immunological memory), recognizing conditions that impair adaptive immunity (e.g., HIV/AIDS, immunosuppressive therapy post-transplant surgery), assessing patient history of infections and vaccinations.

Relevance to Surgical Nursing Investigations

Understanding these natural defense mechanisms informs several “special investigations” or assessments in surgical nursing:

• Hematological Investigations:

  • Complete Blood Count (CBC): Elevated white blood cell count (leukocytosis), particularly neutrophils, can indicate bacterial infection (innate response). Lymphocytosis might suggest viral infection (adaptive response).

  • Differential White Blood Cell Count: Provides a breakdown of different types of WBCs, offering clues about the type of infection or inflammatory process.

• Inflammatory Markers:

  • C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR): Non-specific markers elevated during inflammation and infection. Useful for monitoring the severity of the inflammatory response post-surgery or in suspected infections.

• Microbiological Investigations:

  • Wound Swabs, Blood Cultures, Urine Cultures: Identify specific pathogens that have breached the body’s defenses, guiding targeted antimicrobial therapy.

• Immunological Investigations:

  • Immunoglobulin Levels: Assessing levels of antibodies (e.g., IgG, IgM) can provide insight into the patient’s humoral immunity.

  • Lymphocyte Counts: Assessing T cell and B cell counts can indicate the status of cell-mediated and humoral immunity, particularly relevant in immunocompromised patients.

• Assessment of Barrier Integrity:

  • Skin and Wound Assessment: Direct observation of surgical incisions and surrounding skin for signs of infection (redness, swelling, warmth, drainage) indicates a failure of the first line of defense and activation of the second.

  • Mucous Membrane Assessment: Checking for signs of oral thrush or respiratory congestion can indicate compromised mucosal defenses.