## **Core Concept**
Major Histocompatibility Complex (MHC) class II molecules are primarily involved in the adaptive immune response. They are responsible for presenting peptide fragments derived from extracellular pathogens to CD4+ T lymphocytes. MHC II molecules are mainly expressed on the surface of antigen-presenting cells (APCs).

## **Why the Correct Answer is Right**
The correct answer includes cells known to express MHC class II molecules and function as antigen-presenting cells. **Macrophages (a)** and **Dendritic cells (b)** are professional APCs that engulf and process pathogens, then present peptide fragments on MHC II to T-cells. **Lymphocytes (c)**, specifically B lymphocytes, can also act as APCs, especially when they recognize antigens via their surface immunoglobulins. These cells play critical roles in initiating immune responses.

## **Why Each Wrong Option is Incorrect**
- **Option D: Eosinophils** are primarily involved in combating parasitic infections and in allergic reactions, particularly through the release of granules that damage the parasites or contribute to inflammation. They are not typically considered professional antigen-presenting cells.
- **Option E: Platelets** are anucleate cell fragments involved in blood clotting and do not participate in antigen presentation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that MHC class II molecules are crucial for the activation of CD4+ T helper cells, which then coordinate the immune response, including activating B cells and cytotoxic T cells. Professional APCs like dendritic cells, macrophages, and B cells are the primary cells expressing MHC II.

## **Correct Answer:** B. a, b, c.