Pathology

Fundamentals of the Immune System

  • Definition: Derived from immunis, representing physiological mechanisms that allow an individual to recognize foreign materials and neutralize, eliminate, or metabolize them.

  • Components of the Immune Response:

    • Humoral immunity (antibody production by B-cells)

    • Cellular immunity (activation of cellular components)

    • Activation of white blood cells and tissue macrophages

    • Activation of serum complement proteins leading to lysis and death of foreign cells

  • Core Functions: Defense, homeostasis, and surveillance.

  • Influencing Factors: Genetic makeup, age, metabolic health, and environment.

Immune Anatomy and Organs

Organs of the Immune System

  • Primary and Secondary Lymphoid Structures:

    • Thymus: Mediates T-cell maturation and differentiation.

    • Bone Marrow: Primary site of hematopoiesis and B-cell maturation.

    • Lymph Nodes: Encapsulated structures that filter lymphatic fluid and house lymphocytes and macrophages.

    • Spleen: Filters blood-borne antigens and clears aged erythrocytes.

    • Tonsils: Mucosal-associated lymphoid tissue protecting the respiratory and oral portals.

Immunoglobulins (Antibodies)

  • IgG:

    • Most abundant immunoglobulin class; found in serum and amniotic fluid.

    • Serves as the principal defender against bacteria, viruses, and toxins.

  • IgA:

    • Secretory antibody found in body fluids including tears, saliva, gastrointestinal secretions, and respiratory tract secretions.

  • IgM:

    • Confined primarily to the intravascular space (bloodstream).

    • First antibody produced in response to primary antigen exposure.

  • IgE:

    • Mediates immediate hypersensitivity and allergic reactions; provides defense against parasitic infestations.

  • IgD:

    • Present in serum and umbilical cord blood; acts as a B-cell surface antigen receptor.

Inflammation and Infection Principles

  • Definitions:

    • Infection: Invasion and multiplication of pathogenic microorganisms within host tissue and the subsequent biological reaction to organisms and toxins.

    • Inflammation: Immediate protective biological response of vascular tissues to injury or cellular necrosis.

  • Purposes of Inflammation: Confining injurious agents, increasing local delivery of plasma proteins and leukocytes, amplifying immune response, destroying foreign materials, and facilitating tissue repair.

  • Cardinal Signs of Inflammation:

    • Rubor: Redness

    • Calor: Heat

    • Dolor: Pain

    • Tumor: Swelling

    • Functio Laesa: Loss or alteration of function

  • Chain of Infection: Infectious Agent →\rightarrow Reservoir →\rightarrow Portal of Exit →\rightarrow Mode of Transmission →\rightarrow Portal of Entry →\rightarrow Susceptible Host.

  • Transmission Pathways: Direct and indirect contact, airborne/droplet, vehicles (utensils, towels), and biological/mechanical vectors (mosquitoes, flies).

Transmission Precautions and Clinical Assessment

  • Physical Assessment Priorities:

    • Inspection: Lesion distribution, cutaneous rashes, drainage characteristics from orifices or wounds, sputum color and consistency, and body temperature.

    • Palpation: Assessment of lymphadenopathy and cutaneous warmth.

    • Auscultation: Lung sounds, blood pressure, and heart tones.

  • Isolation Precautions Categories:

    • Strict Isolation: Requires private room, masks, gloves, gown; indicated for Tuberculosis and Rubeola.

    • Enteric Precautions: Requires private room, gloves, gown, hand hygiene; indicated for Hepatitis A and Typhoid fever.

    • Droplet Precautions: Requires private room and masks; indicated for Rubella, Pneumonia, and Meningitis.

    • Blood and Body Fluid Precautions: Requires private room and barrier protection against exposure; indicated for Hepatitis B.

    • Reverse (Protective) Isolation: Requires private room and strict aseptic precautions; indicated for severe immunosuppression (AIDS, advanced malignancy).

Pediatric and Viral Infectious Diseases

  • Infection Progression Stages:

    • Incubation Phase: Pathogen entry and replication without overt symptoms.

    • Prodromal Phase: Nonspecific symptoms as the organism disseminates.

    • Acute Phase: Maximum severity of host-pathogen interaction and clinical symptoms.

    • Recovery (Convalescent) Phase: Subsidence of symptoms and physiological restitution.

  • Roseola Infantum:

    • Affects children 6 months to 3 years6\text{ months to }3\text{ years}; etiology unknown.

    • Incubation: 8–14 days8\text{--}14\text{ days} (mean: 10 days10\text{ days}).

    • High fever up to 103∘F–40∘C103^\circ\text{F}\text{--}40^\circ\text{C} lasting 3–5 days3\text{--}5\text{ days}, abruptly falling as a rose-colored maculopapular rash appears.

    • Management: Antipyretics (Paracetamol for temperatures >38.5∘C> 38.5^\circ\text{C}), lukewarm sponge baths without shivering, fluid support.

  • Rubella (German Measles):

    • Agent: Myxovirus; spread via direct contact.

    • Incubation: 14–21 days14\text{--}21\text{ days}; communicability spans 7 days7\text{ days} prior to 5 days5\text{ days} following rash onset.

    • Manifestations: Rose-colored macules coalescing into pale/rose-red papules, suboccipital and retroauricular lymphadenopathy, low-grade fever, cough; rash disappears by day 44.

  • Measles (Rubeola):

    • Agent: Paramyxovirus; incubation 8–10 days8\text{--}10\text{ days}; transmitted during prodromal phase.

    • Pre-eruptive stage (3–6 days3\text{--}6\text{ days}): High fever, coryza, purulent conjunctivitis, and pathognomonic Koplik spots (bluish-white specks with red areola on buccal mucosa).

    • Eruptive stage: Confluent cephalocaudal maculopapular rash starting at hairline and descending downward; subsequent fine desquamation.

  • Varicella (Chickenpox):

    • Agent: Varicella zoster virus; centripetal rash distribution.

    • Lesion sequence: Macules →\rightarrow papules →\rightarrow clear vesicles on an erythematous base →\rightarrow pustules →\rightarrow crusting.

    • Management: Acyclovir, calamine lotion, daily baths, isolation, secondary infection control with Penicillin.

  • Herpes Zoster (Shingles):

    • Reactivation of latent Varicella zoster virus residing in dorsal sensory ganglia due to waning antibody titers.

    • Unilateral dermatomal eruption of vesicular clusters with severe neuralgic burning pain along affected sensory nerves.

  • Smallpox (Variola):

    • Filterable virus; incubation 7–16 days7\text{--}16\text{ days} (12 days12\text{ days} average).

    • Abrupt onset of high fever, prostrating backache, headache, and centrifugal rash covering face, arms, and legs; fever remits with rash emergence.

  • Poliomyelitis:

    • Enterovirus transmitted via fecal-hand-oral route; selectively destroys anterior horn motor neurons in the spinal cord.

    • Minor illness: Nonspecific prodromal symptoms (1–3 days1\text{--}3\text{ days} prior to paralysis).

    • Major illness: Aseptic meningitis, nuchal rigidity, asymmetrical flaccid muscle weakness, calcaneocavus deformity (gastrocnemius weakness), claw toes.

  • Rabies:

    • Rhabdovirus encephalomyelitis transmitted via animal bite saliva; uniformly fatal once clinical onset occurs.

    • Neurologic stage (2–7 days2\text{--}7\text{ days}): Hydrophobia, pharyngeal spasms, agitation, autonomic instability, hypersalivation, rapidly progressive encephalitis leading to coma, disseminated intravascular coagulation (DIC), and cardiac arrest.

    • Immediate Wound Management: Copious washing under running pressure with soap and water for 5–10 minutes5\text{--}10\text{ minutes}, encouraging mild bleeding to flush virus without scrubbing; subsequent topical antiseptic application.