All of these are antigen presenting cells (APC’s)except -a) T cellsb) B cellsc) Fibroblastsd) Dendritic cellse) Langerhans cells

**Core Concept**
Antigen-presenting cells (APCs) are a group of immune cells that play a crucial role in initiating and regulating the adaptive immune response. They capture, process, and present antigens to T cells, thereby activating an immune response. APCs are essential for the recognition and elimination of pathogens and foreign substances from the body.

**Why the Correct Answer is Right**
The correct answer is T cells, as they are not traditional antigen-presenting cells. T cells can, however, act as professional APCs under certain conditions, such as in the presence of certain cytokines or during viral infections. T cells recognize antigens presented by APCs, but they do not express MHC class II molecules, which are essential for antigen presentation. Dendritic cells, Langerhans cells, and fibroblasts are all professional APCs that express MHC class II molecules and can present antigens to T cells.

**Why Each Wrong Option is Incorrect**
* **Option B:** B cells are professional APCs that express MHC class II molecules and can present antigens to T cells. They are capable of antigen presentation, making them incorrect as an exception.
* **Option C:** Fibroblasts can act as APCs, especially in the context of wound healing, where they express MHC class II molecules and present antigens to T cells. They are not traditional APCs but can function as such under certain conditions.
* **Option D:** Dendritic cells and Langerhans cells are both professional APCs that express MHC class II molecules and present antigens to T cells. They are essential for initiating the adaptive immune response and are not exceptions to the rule.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that APCs can be broadly categorized into professional and non-professional APCs. Professional APCs, such as dendritic cells, Langerhans cells, and B cells, are highly efficient at antigen presentation, whereas non-professional APCs, such as fibroblasts and endothelial cells, can also present antigens but with less efficiency.

**Correct Answer:** A. T cells