Cell markers, also known as cell surface antigens, serve as monograms to help identify and classify cells. The majority of them are molecules or antigens within cell's plasma membrane. Unique to different cell types, there exist specific combinations of markers or antigens.

Antigens are substances that are recognized by the immune system as foreign and can trigger an immune response. They can be present in various locations, but the correct answer to this question is option D, indicating that antigens are present on the cell surface.

Explanation:

Antigens can be classified into two main categories based on their location:

1. Endogenous antigens: These are antigens that are produced inside the cell and are presented on the cell surface in association with major histocompatibility complex (MHC) molecules. Endogenous antigens can include:

- Viral or bacterial proteins produced by infected cells
- Tumor antigens produced by cancer cells
- Self-proteins that are aberrantly expressed or modified

2. Exogenous antigens: These are antigens that are taken up by specialized cells called antigen-presenting cells (APCs), processed, and presented on the cell surface in association with MHC molecules. Exogenous antigens can include:

- Microbes, such as bacteria or viruses, that are engulfed by phagocytic cells
- Toxins or other foreign substances that enter the body

In both cases, the antigens are presented on the cell surface in association with MHC molecules, which help to activate T cells and initiate an immune response.

Therefore, the correct answer to this question is option D, indicating that antigens are present on the cell surface. This is where they are recognized by T cells and other components of the immune system, leading to the activation of an immune response.

Basically antigens are present on the cell surface.
Your immune system “sees” foreign invaders, like viruses, by looking at antigens produced from the proteins in those viruses. Antigens are often found on the surface of cells. When a virus infects a cell, its proteins get cut up and “displayed” on the surface of the cell for the immune system to see.