Why We Rarely Fall Sick 🌟

Your body owns an incredible defence unit called immunity—the power to spot and smash disease-causing agents before they win the battle :contentReference[oaicite:0]{index=0}.

1. Innate Immunity 🛡️ (built-in from birth)

  • Physical barriers: Skin and sticky mucus line every entry point, trapping microbes on the spot :contentReference[oaicite:1]{index=1}.
  • Physiological barriers: Stomach acid, saliva, and tears create harsh conditions that germs hate :contentReference[oaicite:2]{index=2}.
  • Cellular barriers: Neutrophils, monocytes, natural-killer cells, and tissue macrophages gobble up invaders 🍽️ :contentReference[oaicite:3]{index=3}.
  • Cytokine barrier: Virus-infected cells release interferons to warn and shield neighbouring cells 📢 :contentReference[oaicite:4]{index=4}.

2. Acquired Immunity 🧠 (smart & specific)

When a new pathogen arrives, your body fires a primary response. Meet the same germ again and a turbo-charged secondary (anamnestic) response kicks in—thanks to immune memory :contentReference[oaicite:5]{index=5}.

Key Players

  • B-lymphocytes: Craft protein weapons called antibodies to neutralise pathogens.
  • T-lymphocytes: Coach B cells and lead the cell-mediated response (CMI) to destroy infected cells :contentReference[oaicite:6]{index=6}.

Antibody Cheat Sheet 🔬

Each antibody contains two heavy (H) and two light (L) chains—think of it as \(H_2L_2\) arranged like a “Y”. Major classes: IgA, IgM, IgE, IgG :contentReference[oaicite:7]{index=7}.

3. Active vs Passive Immunity ⚔️

  • Active immunity: Your own cells build antibodies after infection or vaccination; slower but long-lasting :contentReference[oaicite:8]{index=8}.
  • Passive immunity: Ready-made antibodies give instant help—e.g., colostrum IgA for newborns or anti-tetanus shots 💉 :contentReference[oaicite:9]{index=9}.

4. Vaccination 💉

Vaccines introduce harmless antigens (or weakened microbes). Memory B and T cells remember the foe, so future encounters end quickly—modern biotech even makes hepatitis B vaccine in yeast! :contentReference[oaicite:10]{index=10}

5. Allergies 🤧

Allergens (dust mites, pollen, animal dander) trigger IgE antibodies; mast cells then release histamine and serotonin—leading to sneezing, watery eyes, and wheezing. Antihistamines and steroids calm the storm :contentReference[oaicite:11]{index=11}.

6. Auto-immunity 🔄

Sometimes defence troops get confused and attack self-cells, causing disorders like rheumatoid arthritis :contentReference[oaicite:12]{index=12}.

7. Your Body’s Immune Headquarters 🏰

Primary lymphoid organs

  • Bone marrow: Birthplace of all blood cells, including lymphocytes.
  • Thymus: “School” for T-cells; large in childhood, shrinks by puberty :contentReference[oaicite:13]{index=13}.

Secondary lymphoid organs

  • Spleen: Bean-shaped filter packed with lymphocytes and phagocytes.
  • Lymph nodes: Tiny checkpoints that trap antigens in lymph.
  • Tonsils, Peyer’s patches, appendix, and MALT: Guard all major entry tracts; MALT alone forms ~50 % of total lymphoid tissue :contentReference[oaicite:14]{index=14}.

High-Yield NEET Nuggets ⭐

  1. Contrast innate vs acquired immunity, and remember the four innate barriers.
  2. Structure of antibodies (\(H_2L_2\)) plus the roles of IgA, IgM, IgE, IgG.
  3. Active vs passive immunity—examples like vaccinations, colostrum, antitoxins.
  4. Mechanism of allergies (IgE, histamine) and quick treatments.
  5. Locations and jobs of primary & secondary lymphoid organs (bone marrow, thymus, lymph nodes, spleen, MALT).