🔬 1. What is Serum?
- Serum is the fluid portion of blood left after clotting.
- It contains antibodies, electrolytes, hormones, antigens, and other proteins — but no blood cells or clotting factors.
🧫 2. Antigen-Antibody Reactions
- Central to serology is the specific binding between antigens and antibodies.
- These reactions are highly specific and form the basis of diagnostic tests.
| Term | Definition |
|---|---|
| Antigen | A foreign substance that induces an immune response |
| Antibody | A protein produced by B-cells in response to an antigen |
⚗️ 3. Types of Serological Reactions
a. Precipitation
- Soluble antigen + antibody → visible precipitate
- Example: Ouchterlony double diffusion
b. Agglutination
- Particulate antigen + antibody → clumping
- Examples:
- Blood typing
- Widal test (for typhoid)
c. Neutralization
- Antibody neutralizes the effect of a toxin or virus
- Example: Antitoxin assays
d. Complement Fixation Test (CFT)
- Used to detect presence of specific antibodies by their ability to fix complement
e. Enzyme-Linked Immunosorbent Assay (ELISA)
- Highly sensitive test using enzyme-labeled antibodies
- Detects antigens or antibodies (e.g., HIV, Hepatitis B)
f. Western Blot
- Detects specific proteins (antibodies or antigens) using electrophoresis and labeled antibodies
g. Rapid Immunochromatographic Tests
- Lateral flow tests (e.g., pregnancy test, COVID-19 rapid test)
🧬 4. Types of Antibodies Detected
| Antibody | Function | Diagnostic Relevance |
|---|---|---|
| IgM | First antibody produced in infection | Indicates recent infection |
| IgG | Long-term immunity | Indicates past exposure or immunity |
| IgA | Mucosal immunity | Seen in secretions like saliva, tears |
| IgE | Allergic responses | Elevated in parasitic infections |
| IgD | B-cell receptor | Less commonly tested |
🧪 5. Common Serological Tests
| Test | Disease Detected |
|---|---|
| Widal Test | Typhoid fever |
| ELISA | HIV, Hepatitis B, COVID-19 |
| VDRL/RPR | Syphilis |
| ASO Titer | Streptococcal infections |
| CRP | Inflammation marker (not specific to infection) |
| HCV Antibody Test | Hepatitis C |
| Dengue IgM/IgG | Dengue fever |
🧑⚕️ 6. Clinical Applications of Serology
- Infectious Disease Diagnosis (HIV, syphilis, hepatitis, dengue)
- Blood Typing & Transfusion Safety
- Immunity Assessment (e.g., rubella or hepatitis B vaccination response)
- Autoimmune Diseases (ANA in lupus)
- Monitoring Disease Progression (e.g., HIV viral load)
🧷 7. Limitations of Serology
- Window Period: Time between infection and antibody development (false negatives)
- Cross-Reactivity: Antibodies may react with similar antigens (false positives)
- Requires Confirmation: Positive ELISA often followed by confirmatory tests (e.g., Western blot)
📘 Summary Table
| Feature | Description |
|---|---|
| Target | Antigen or antibody in serum |
| Sample | Blood serum, plasma, body fluids |
| Main Techniques | ELISA, agglutination, precipitation |
| Use | Diagnosis, screening, monitoring |