Here's why:
* Clonal Selection: This process describes how the immune system selects and expands specific lymphocytes (B cells and T cells) that recognize a particular antigen. This happens through a series of steps:
* Antigen Recognition: Lymphocytes with receptors that bind to the antigen are activated.
* Clonal Expansion: Activated lymphocytes proliferate, creating clones of themselves.
* Differentiation: Some clones differentiate into effector cells that directly combat the antigen, while others become memory cells that provide long-term immunity.
* Role of APCs: APCs are crucial for initiating clonal selection. They:
* Capture antigens: APCs engulf and process antigens from pathogens or other foreign substances.
* Present antigens: They present processed antigen fragments on their surface, specifically on MHC (major histocompatibility complex) molecules. This presentation allows T cells to recognize the antigen.
* Activate T cells: When a T cell recognizes its specific antigen presented by an APC, it becomes activated and initiates the clonal selection process.
Examples of APCs:
* Macrophages
* Dendritic cells
* B cells (to a lesser extent)
So, while lymphocytes are the central players in clonal selection, APCs are essential for initiating the process by presenting the antigen to the lymphocytes.
