Q1. Source: CSIR NET, November 2020, Session II
An antigen was injected into a mouse. Macrophages and antigen-primed TH cells were isolated from the mouse to perform the following in vitro experiments:
A. Macrophages were treated with the antigen for an hour and then incubated with TH cells.
B. Macrophages were treated with paraformaldehyde first and then treated with the antigen for an hour. These macrophages were then incubated with TH cells.
C. Macrophages were treated with paraformaldehyde first and then treated with the digested (proteolytically cleaved) antigen for an hour. These macrophages were then incubated with TH cells.
D. Macrophages were treated with the antigen for an hour and then treated with paraformaldehyde. These macrophages were then incubated with TH cells.
Which of the above experiments would lead to TH cell proliferation?
a. A and D only
b. B only
c. A, C, and D only
d. C and D only
Before analyzing the options, remember that macrophages activate helper T cells only after processing an antigen and presenting antigenic peptides on MHC Class II molecules.
Paraformaldehyde (PFA) fixes cells by cross-linking proteins and effectively halts cellular processes such as phagocytosis, antigen processing, and cytokine secretion.
Statement Analysis
A. Macrophages treated with antigen first
The macrophage remains metabolically active and can internalize, process, and present the antigen.
When incubated with antigen-primed TH cells, the peptide-MHC complex is recognized, leading to TH cell activation and proliferation.
TH cells proliferate.
B. Macrophages fixed with paraformaldehyde before antigen exposure
The moment paraformaldehyde is added, antigen uptake and processing stop.
Since the macrophage can no longer process the antigen, no peptide-MHC complexes are generated.
Without antigen presentation, TH cells cannot be activated.
TH cells do not proliferate.
C. Macrophages fixed first, then exposed to proteolytically cleaved antigen
Although antigen processing machinery is inactive, the antigen has already been proteolytically cleaved into peptides.
These peptides can directly associate with available MHC molecules on the macrophage surface without requiring intracellular processing.
Therefore, antigen presentation can still occur.
D. Macrophages exposed to antigen before fixation
Antigen processing occurs while the macrophage is still active.
By the time paraformaldehyde is added, peptide-MHC complexes have already formed.
Fixation prevents further processing but does not remove the already-presented antigen.
TH cells can still recognize the antigen and proliferate.
Correct Answer: c. A, C, and D only
Exam Insight
A common trap is assuming that paraformaldehyde completely prevents T-cell activation.
Fixation prevents antigen processing, not antigen recognition.
If peptide-MHC complexes are already present or if pre-digested peptides are provided, T-cell activation can still occur.
Q2. Source: GATE 2020
The human major histocompatibility complex (MHC) is:
a. Polygenic and monomorphic
b. Polygenic and polymorphic
c. Monogenic and polymorphic
d. Monogenic and monomorphic
Understanding the Terminology
Monogenic vs Polygenic
- Monogenic: Controlled by a single gene.
- Polygenic: Controlled by multiple genes.
The human MHC consists of several genes located on chromosome 6. Examples include:
HLA-A, HLA-B, HLA-C, HLA-DP, HLA-DQ, HLA-DR
Since multiple genes contribute to MHC expression, it is polygenic.
Monomorphic vs Polymorphic
- Monomorphic: Exists in only one form.
- Polymorphic: Exists in many allelic variants.
MHC genes are among the most polymorphic genes in the human genome.
Different individuals possess different MHC alleles, which increases the diversity of antigen presentation within a population.
This diversity helps populations respond to a wider range of pathogens.
Therefore, MHC is polymorphic.
Correct Answer: b. Polygenic and Polymorphic