While studying pathogenic bacteria, a protein with the following features was identified:

A. It was secreted during infection conditions, but not in vitro cultures
B. It was also observed to be present in the membranous fraction in traces, which was released upon bacterial lysis
C. It had a heat labile N-terminal enzymatic domain that binds MHC molecules, stimulating T cells non-specifically
D. It had a C-terminal non-enzymatic domain which was highly antigenic and heat-stable

How will you best classify the toxic nature of this protein?

Correct option is B

Explanation-

Option a - Endotoxin
Incorrect – Endotoxins are lipopolysaccharides (LPS) from Gram-negative bacterial outer membranes, not proteins. They don't bind MHC or TCRs.

Option b - Superantigen
Correct – Matches all described features: secreted protein, binds MHC, stimulates T cells non-specifically, heat-labile/stable domains.

Option c -  Pore-forming toxin
Incorrect – These disrupt host cell membranes, not MHC/TCR interaction. No relevance to T cell activation.

Option d - AB toxin
Incorrect – These have distinct A (active) and B (binding) subunits (e.g., diphtheria, cholera toxins), but do not act via MHC or non-specific T cell stimulation.

So, the correct answer is option b – Superantigen