ELISA and Clonal Selection of B-lymphocytes

Direct ELISA

Purpose: To detect a specific antigen (like a virus protein) in a sample.

🧪 Steps:

1. Antigen is attached directly to the bottom of the well.

2. Enzyme-linked antibody (specific to the antigen) is added.

3. Antibody binds directly to the antigen.

4. Substrate is added → if enzyme is present, color develops.

✅ Pros:

Simple and fast (fewer steps).

Less chance of cross-reactions.

(a) Clonal Selection of B-Lymphocytes (13 marks)

Clonal selection is how the immune system chooses the right B cell to fight a specific germ (antigen).

🔹 Steps:

1. Every B cell has a unique antibody (BCR) on its surface.

2. When a matching antigen enters the body, it binds to that specific B cell.

3. This activates the B cell.

4. With help from a helper T cell, the B cell multiplies (clonal expansion).

5. Some become plasma cells that make lots of antibodies.

6. Some become memory cells for future protection.

🧠 Key idea: Only B cells that match the antigen get activated — like finding the right key for a lock.