Which type of immune cell is activated by the presence of an antigen?

• A. T cells
• B. Red blood cells
• C. Platelets
• D. Endothelial cells

Answer: (A)

The activation of immune cells by the presence of an antigen is a fundamental aspect of the adaptive immune response. T cells, which are a type of lymphocyte, play a critical role in recognizing and responding to specific antigens. When an antigen is present, it is processed and presented by antigen-presenting cells (APCs), such as dendritic cells or macrophages. These APCs display fragments of the antigen on their surface using molecules called Major Histocompatibility Complex (MHC) proteins.

T cells possess T-cell receptors (TCRs) that specifically bind to these MHC-antigen complexes. This interaction is what activates T cells, prompting them to proliferate and differentiate into various subsets such as helper T cells and cytotoxic T cells. Helper T cells assist other immune cells, while cytotoxic T cells directly kill infected or cancerous cells.

In contrast, red blood cells, platelets, and endothelial cells do not have a role in the adaptive immune response. Red blood cells primarily function in oxygen transport, platelets are involved in clotting, and endothelial cells line blood vessels and do not recognize antigens. Therefore, the specific activation of immune cells by antigens is a clear indication of the role of T cells in the