Apart from the interaction of antigenic peptide with the TCR-CD3 complex (signal 1), T-cell activation requires another signal termed as “Co-stimulatory signal” (signal 2). Given below are a few statements regarding the co-stimulatory signal.
A. All three professional antigen-presenting cells (dendritic cells, macrophages, and B cells) possess the same capacity in respect to delivering the co-stimulatory signal.
B. Co-stimulatory signals are not antigen-specific.
C. The principal co-stimulatory molecules expressed on antigen-presenting cells are B7-1 and B7-2.
D. Unlike B7, CD28, the ligand for B7 which is expressed on T cells, is not a member of the immunoglobulin superfamily.
Which of the above statement(s) is/are NOT true?

Correct option is C

Explanation:

Let's break down the statements:

A. All three professional antigen-presenting cells (dendritic cells, macrophages, and B cells) possess the same capacity in respect to delivering the co-stimulatory signal.

• This statement is incorrect. While all three types of professional antigen-presenting cells (APCs) can deliver co-stimulatory signals, dendritic cells are generally the most efficient at providing these signals. Dendritic cells are the most potent APCs for initiating T cell responses. Macrophages and B cells can also deliver co-stimulatory signals, but dendritic cells do it more effectively in the context of T cell activation. Hence, the statement is incorrect.

B. Co-stimulatory signals are not antigen-specific.

• This statement is correct. Co-stimulatory signals, like those provided by B7 molecules on APCs, are independent of the antigen that is being presented. These signals are necessary for full T cell activation, regardless of the antigen. They act as a "second signal" to confirm the presence of a pathogen or other threat.

C. The principal co-stimulatory molecules expressed on antigen-presenting cells are B7-1 and B7-2.

• This statement is correct. The principal co-stimulatory molecules on APCs are B7-1 (CD80) and B7-2 (CD86). These molecules interact with CD28 on T cells to provide the essential co-stimulatory signals for full T cell activation.

D. Unlike B7, CD28, the ligand for B7 which is expressed on T cells, is not a member of immunoglobulin superfamily.

• This statement is incorrect.CD28, which binds to B7 molecules on APCs, is a member of the immunoglobulin superfamily. CD28 contains Ig-like domains that allow it to interact with the B7 molecules on APCs. So, the statement is incorrect.

Conclusion:

• Statement A is incorrect because dendritic cells are much more efficient in delivering co-stimulatory signals compared to other APCs like macrophages and B cells.

• Statement D is incorrect because CD28 is indeed a member of the immunoglobulin superfamily.

• Statements B and C are correct.

Thus, the correct answer is Option 3: A and D, as these are the statements that are NOT true.