Question

Mixed lymphocyte culture is used to identify:

• A. MHC class I antigen
• B. MHC class II antigen
• C. B lymphocytes
• D. T helper cells

Hint:

MLC proliferation is driven by CD4 helper T-cells reacting to an MHC class II mismatch.

Answer 1

The Correct Option is B

Step 1: Recall what a mixed lymphocyte culture (MLC) is.
In this test, lymphocytes from two different people are grown together in the same culture dish. If the two people have different HLA (human leukocyte antigen) types, the T-cells from one person recognize the other person's cells as foreign and start dividing.

Step 2: Understand which cells are doing the recognizing.
The T-cells that respond and divide in this test are mainly CD4-positive helper T-cells. These cells do not recognize MHC class I molecules well on their own, they are built to recognize MHC class II molecules, the ones normally found on antigen-presenting cells like B-cells, macrophages, and dendritic cells.

Step 3: Connect this to what the test measures.
Because the degree of T-cell proliferation in the mixture depends on how different the two people's MHC class II molecules (mainly HLA-DR) are, the amount of proliferation is used as a functional readout of MHC class II compatibility. This makes the mixed lymphocyte culture a classic method to check MHC class II matching, historically used before organ or bone marrow transplants.

Step 4: Rule out the other options.

(1) MHC class I antigen: MHC class I differences are best picked up by cytotoxic T-cell assays, not by the proliferative mixed lymphocyte reaction.

(3) B lymphocytes: B-cells are present in the culture as antigen-presenting cells, but the test is not designed to identify or count them.

(4) T helper cells: These are the cells doing the responding in the assay, but the test is used to identify differences in MHC class II, not to identify the T-helper cells themselves.

Final Answer:
Mixed lymphocyte culture is used to detect differences in MHC class II antigens.
\[ \boxed{\text{MHC class II antigen}} \]