Domain 2.11Anatomy and Physiology

Immune System And Common Diseases

Last updated June 16, 2026

Overview

Welcome to your study guide for the Immune System! This system is the body’s "Department of Defense." It is a complex network of cells, tissues, and organs that work together to defend the body against attacks by "foreign" invaders (bacteria, parasites, fungi, and viruses).

Why This Matters for Medical Assistants:

  • Vaccinations: You will be responsible for administering immunizations to help patients build immunity.
  • Infection Control: You must understand how the immune system works to practice proper asepsis and prevent the spread of nosocomial infections.
  • Emergency Response: Recognizing the signs of a severe allergic reaction (anaphylaxis) is a life-saving skill.
  • Lab Work: You will often perform or prepare tests that measure immune function, such as CLIA-waived Mono tests or drawing blood for white blood cell counts.

1. Anatomy and Physiology of the Immune System

The immune system is closely linked to the lymphatic system. They share organs and pathways to filter blood and fight infection.

Key Organs and Structures

Structure Location/Function Clinical Note
Lymph Nodes Small, bean-shaped structures found throughout the body (neck, axilla, groin). They filter lymph and trap pathogens. Swollen nodes (lymphadenopathy) often indicate an active infection.
Spleen Located in the Left Upper Quadrant (LUQ). It filters blood, removes old red blood cells, and activates lymphocytes. Because the spleen is highly vascular, trauma to the LUQ can cause severe internal bleeding.
Thymus Located posterior to the sternum. It is the site where T-cells mature. It is largest in children and shrinks (atrophies) in adulthood.
Bone Marrow Found inside bones. Produces all blood cells, including White Blood Cells (Leukocytes).

The "Soldiers": White Blood Cells (Leukocytes)

White Blood Cells (WBCs) are the primary defenders. A "Differential" count measures the percentage of each type.

Mnemonic: To remember the abundance of WBCs from most to least, think: Never Let Monkeys Eat Bananas.

  1. Neutrophils: The "first responders." They fight Bacterial infections.
  2. Lymphocytes: The "special forces." They fight Viral infections and produce antibodies.
    • B-Cells: Produce antibodies.
    • T-Cells: Destroy infected cells directly (Killer T-cells) or help direct the immune response (Helper T-cells/CD4).
  3. Monocytes: Clean up debris (phagocytosis) and fight chronic infections.
  4. Eosinophils: Elevated during Allergic reactions and Parasitic infections.
  5. Basophils: Release histamine (inflammation) and heparin (blood thinner).

Types of Immunity (High-Yield Topic!)

You must understand how a patient acquires immunity.

Type Definition Source/Example
Innate Immunity The defense you are born with. Non-specific. Skin, mucus, stomach acid, inflammation.
Adaptive Immunity Specific defense developed over time. Involves memory cells. Antibodies targeting a specific flu virus.

Classifications of Acquired Immunity:

Classification Mechanism Example
Natural Active You get sick, your body fights it, and you remember it. Contracting Chickenpox and recovering.
Artificial Active You are given a "fake" fight to train your body. Vaccines (Flu shot, MMR).
Natural Passive Antibodies are given to you naturally by someone else. Breastfeeding or antibodies crossing the placenta from mother to fetus.
Artificial Passive You are injected with pre-made antibodies. Antivenom for a snake bite or immune globulin injections.

2. Common Diseases and Conditions

Anaphylaxis (Anaphylactic Shock)

A severe, life-threatening, systemic allergic reaction. This occurs when the immune system overreacts to a trigger (allergen), releasing a flood of chemicals (histamine) that cause the body to go into shock.

  • Pathophysiology: Massive vasodilation (widening of blood vessels) causes blood pressure to drop significantly. Bronchoconstriction (narrowing of airways) makes breathing difficult.
  • Key Signs and Symptoms:
    • Dyspnea (difficulty breathing), wheezing, stridor.
    • Urticaria (hives) and swelling of the lips/tongue (angioedema).
    • Hypotension (low blood pressure) and tachycardia (rapid heart rate).
  • Common Risk Factors: History of allergies to bee stings, peanuts, shellfish, latex, or medications (e.g., Penicillin).
  • Treatment Highlights:
    • Immediate intervention is required.
    • Epinephrine (EpiPen): The first-line drug. It constricts blood vessels (raising BP) and relaxes airway muscles.
    • MA Role: Notify the provider immediately, ensure the emergency crash cart is ready, and assist with vitals/oxygen.

Exam Tip: If a patient reports an allergy, always ask about the reaction. "Stomach upset" is a side effect; "Throat closing" is an allergy.

Systemic Lupus Erythematosus (SLE or Lupus)

A chronic autoimmune disease where the immune system attacks healthy tissue throughout the body, including joints, skin, kidneys, and blood cells.

  • Pathophysiology: The body produces autoantibodies that target its own DNA and cell nuclei, causing widespread inflammation and tissue damage.
  • Key Signs and Symptoms:
    • Butterfly Rash: A classic red rash across the cheeks and bridge of the nose (malar rash).
    • Fatigue and fever.
    • Joint pain and swelling (arthritis).
    • Raynaud’s phenomenon (fingers turn white/blue in cold).
  • Common Risk Factors: Women of childbearing age are most affected. Genetics and environmental triggers (sunlight, stress) play a role.
  • Key Diagnostic Tests:
    • Antinuclear Antibody (ANA): A positive result suggests an autoimmune disorder.
    • ESR/CRP: Elevated levels indicate inflammation.
  • Treatment Highlights:
    • NSAIDs for pain/inflammation.
    • Corticosteroids (Prednisone) to suppress the immune system.
    • MA Role: Educate patients on sun protection (photosensitivity) and monitoring blood pressure (kidney involvement).

Mnemonic: LUPUS = Likely Unexplained Pain, Under Skin (rash). Remember the "Wolf" mask (Lupus is Latin for wolf) describing the butterfly rash.

Rheumatoid Arthritis (RA)

A chronic inflammatory disorder affecting many joints, including those in the hands and feet.

  • Pathophysiology: Unlike osteoarthritis (wear-and-tear), RA is autoimmune. The immune system attacks the lining of the joints (synovium), causing painful swelling that can eventually result in bone erosion and joint deformity.
  • Key Signs and Symptoms:
    • Morning Stiffness: Joints are stiff for more than 30 minutes after waking up.
    • Symmetry: Symptoms usually occur on both sides of the body (e.g., both wrists).
    • Joint swelling, warmth, and redness.
    • Visible deformities in hands/fingers in late stages.
  • Common Risk Factors: Family history, smoking, and being female.
  • Key Diagnostic Tests:
    • Rheumatoid Factor (RF): Blood test looking for the specific antibody.
    • Erythrocyte Sedimentation Rate (ESR): Measures inflammation.
  • Treatment Highlights:
    • DMARDs (Disease-Modifying Antirheumatic Drugs) like Methotrexate.
    • MA Role: Assist with mobility, encourage low-impact exercise recommendations from the provider.

HIV/AIDS

  • Pathophysiology: The Human Immunodeficiency Virus (HIV) attacks the immune system, specifically the CD4 T-cells (Helper T-cells). Without these, the body cannot fight off infections.
  • AIDS (Acquired Immunodeficiency Syndrome): The final stage of HIV infection. It is diagnosed when the CD4 T-cell count drops below 200 cells/mm³ or when the patient develops an opportunistic infection.
  • Transmission: Blood, semen, vaginal fluids, breast milk. It is NOT spread by casual contact.
  • Clinical Note: Patients are prone to Opportunistic Infections—diseases that normally don't hurt healthy people but are deadly to those with weak immune systems (e.g., Pneumocystis pneumonia).

Infectious Mononucleosis ("Mono")

  • Pathogen: Epstein-Barr Virus (EBV).
  • Transmission: Saliva (often called "The Kissing Disease").
  • Symptoms: Severe fatigue, fever, sore throat, and swollen lymph nodes.
  • Critical Complication: Splenomegaly (enlarged spleen).
  • MA Role: Advise patients to avoid contact sports to prevent rupture of the spleen (located in the LUQ).

3. Diagnostic Tests and Procedures

Allergy Testing

  • Scratch/Skin Prick Test: Small amounts of allergens are applied to the skin (usually the back or forearm).
  • Intradermal Test: Allergen is injected under the skin.
  • Observation: A "wheal" (raised, red bump) indicates a positive reaction.
  • Safety: Emergency equipment (epinephrine/oxygen) must be available in case of anaphylaxis during testing.

Blood Tests for Inflammation

These tests do not diagnose a specific disease but indicate that the immune system is active/inflamed.

  • C-Reactive Protein (CRP): A protein produced by the liver during inflammation.
  • Erythrocyte Sedimentation Rate (ESR or "Sed Rate"): Measures how quickly red blood cells settle. Faster settling = more inflammation.

4. Final Key Takeaways

  • Anaphylaxis is an Emergency: Recognize the signs—wheezing, hives, and hypotension. The treatment is Epinephrine.
  • Lupus (SLE): Look for the Butterfly Rash on the face and joint pain. It is an autoimmune condition.
  • Rheumatoid Arthritis vs. Osteoarthritis: RA is autoimmune, symmetrical, and causes morning stiffness. OA is mechanical wear-and-tear.
  • HIV/AIDS Metric: Diagnosis of AIDS occurs when CD4 count is < 200.
  • The Spleen: Located in the LUQ. It filters blood. Enlargement (from Mono) poses a rupture risk.
  • Vaccines: Provide Artificial Active Immunity.
  • WBC Types:
    • Neutrophils = Bacteria.
    • Lymphocytes = Virus.
    • Eosinophils = Allergies/Parasites.
  • Pediatric Warning: Never give aspirin to a child with a viral infection (Flu/Chickenpox) due to the risk of Reye's Syndrome (liver/brain damage).