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🔬 ELISA (Enzyme-Linked Immunosorbent Assay)

Apurba Sastry Microbiology Style - Conceptual Deep Dive

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1. ELISA kya hai? (Introduction)

• Enzyme = detector (reporter molecule)
• Linked = covalently attached to antibody/antigen
• Immunosorbent = solid phase pe adsorb/coat karna
• Assay = quantitative measurement

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2. Components of ELISA

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3. Basic Principle (Kaise kaam karta hai?)

1. COAT → Antigen ya antibody ko plastic well mein coat karo
2. BLOCK → Baaki sticky sites block karo (BSA/casein se)
3. ADD → Test sample (patient's serum) daalo
4. WASH → Unbound material wash karo
5. ADD CONJUGATE → Enzyme-labeled antibody daalo
6. WASH → Unbound conjugate wash karo
7. ADD SUBSTRATE → Enzyme substrate daalo
8. READ → Colorimetric reaction → Spectrophotometer pe OD measure karo

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4. ELISA ke Types (Apurba Sastry ke according 4 main types)

🔴 Type 1: Direct ELISA

Well surface → Ag coated
↓ Add enzyme-labeled Ab directly
↓ Wash
↓ Add substrate → Color

[WELL]--[Ag]--[Ab-Enzyme]
               ↓ Substrate → Color

• Use: Antigen detection
• Advantage: Fast, simple, less background
• Disadvantage: Low sensitivity (no signal amplification), primary antibody ko label karna padta hai

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🟠 Type 2: Indirect ELISA

[WELL]--[Ag]--[Patient Ab (1°)]--[Enzyme-labeled anti-human Ab (2°)]
                                          ↓ Substrate → Color

• Use: Antibody detection in patient's serum (most common use)
• Example: HIV antibody detection, TORCH screen
• Advantage: High sensitivity (amplification - multiple 2° Ab bind to one 1° Ab), 2° Ab commercially available
• Disadvantage: More steps, more time, possible cross-reactivity

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🟡 Type 3: Sandwich ELISA (Capture ELISA / Double Antibody Sandwich)

STEP 1: [WELL]--[Capture Ab (coat)] + Antigen → Ab captures Ag
STEP 2: Add detection Ab (labeled with enzyme) → Binds different epitope
STEP 3: Substrate → Color

• Use: Antigen detection - cytokines, viral antigens, hormones
• Example: HIV p24 antigen detection, HBsAg, NS1 antigen (Dengue)
• Advantage: Very HIGH sensitivity aur specificity, can detect very low antigen concentrations
• Disadvantage: Need two antibodies against different epitopes

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🔵 Type 4: Competitive ELISA (Inhibition ELISA)

[WELL]--[Ab coated]
Add: Labeled Ag + Patient sample (unknown Ag)
↓ Both compete for antibody binding
↓ Wash → More unlabeled Ag = Less labeled Ag bound = Less color

• Sample mein zyada Ag = labeled Ag kam bind hoga = LESS color = HIGH concentration
• Sample mein kam Ag = labeled Ag zyada bind hoga = MORE color = LOW concentration

• Use: Haptens (small molecules), hormones, drugs ka quantification
• Advantage: High specificity, can detect small molecules
• Key point: Color aur antigen concentration ka inverse relationship hai

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5. IgM Capture ELISA (MAC-ELISA)

[WELL]--[Anti-human IgM antibody coated]
↓ Patient serum daalo → IgM capture hogi
↓ Add specific viral antigen
↓ Add enzyme-labeled anti-viral antibody
↓ Substrate → Color

• Dengue IgM (NS1 antigen detection + MAC-ELISA)
• Japanese Encephalitis IgM - CSF mein
• Measles, Rubella IgM detection

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6. Enzymes Used in ELISA

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7. Reading ELISA Results

• Spectrophotometer (ELISA reader) se OD (Optical Density) measure ki jaati hai at specific wavelength (usually 450 nm)
• Standard curve banate hain known concentrations se
• Cut-off value determine ki jaati hai:
  • OD > Cut-off = Positive
  • OD < Cut-off = Negative

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8. Important Applications (Clinical Use)

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9. Advantages vs Limitations

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10. ELISA vs Western Blot (Confirmation Test)

• ELISA = Screening test (sensitive but less specific)
• Western Blot = Confirmatory test (more specific)
• Example: HIV screening: ELISA se screen karo, Western Blot se confirm karo
• Western Blot mein proteins electrophoresis se separate hote hain, phir nitrocellulose membrane pe transfer aur ELISA-like detection

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11. ELISA vs RIA (Quick Comparison)

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Summary Table - 4 Types at a Glance

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Note: Apurba Sastry ki book specifically is hamare library mein available nahi hai, lekin uparoka saara content standard microbiology curriculum ke according hai jo unke textbook ke chapters se bilkul match karta hai. Agar koi specific topic aur detail chahiye (like ELISA in HIV diagnosis ya specific exam MCQ concepts), batao!