Antigen-processing cell (APC) or Antigen-presenting cell
• T lymphocytes are part of the immune system involved in identifying antigens . However, for an antigen to be recognised by a T-lymphocyte, it must be first processed and "presented" in a form the antigen can recognise. This is the function of an APC, also referred to as accessory cells .
1. An APC (see below for the principal different types) engulfs an antigen. 2. Enzymes in the APC break down the antigen into smaller fragments. 3. These fragments are transported to the surface of the APC, bound with class II MHC (major histocompatibility complex) molecules. 4. A T-cell receptor can now recognise the antigen linked with the MHC and thus binds to it.
Antigen-processing cells include • 1 Macrophages These are large white blood cells that ingest antigens and other foreign substances. Each macrophage contains packets of chemicals and enzymes, which digest the ingested antigen or microbe.
Antigen-processing cells include • 2 Dendritic Cell (DC) Dendritic cells are the principle APC involved in primary immune responses . Their major function is to obtain antigen in tissues, migrate to lymphoid organs and activate T cells . Cell required for, but not actually mediating, a specific immune response.
1 郎格汉斯细胞 (Langerhan’s cells)
上皮组织中的 LC （ 1 ，普通光镜）捕捉外来抗原后即进入引流 淋巴结的 T 细胞区，成为 IDC （ 2 ，扫描电镜照片）。
2 并指状树突细胞（ interdigitating DC ）
IDC 表达高水平的 II 类 MHC 分子和共刺激分子 B7 ，具有激活 T 细胞的能力。
Antigen-processing cells include • 3. B cell