It is the highest point of a series of interactions between macrophages, T lymphocytes, and B lymphocytes.
The infectious agent also known as antigens are ingested and partially destroyed byantigen-presenting cells (APCs), such as macrophages, Langerhans cells, dendritic cells, lymph nodes, and monocytes.
The antigen's fragments then appear on the surface of APCs which are attached to a cell surface glycoprotein known as MHC II (major histocompatibility complex).
There are basically two types of MHC molecules: MHC class I, that are expressed on the surfaces of most cells, and class II, that are expressed only on APCs' surfaces.
The antigen-MHC II complex permits Th cells to bind to the APC, leading to a rapid division of Th cells and the release of cytokine.
T cells then bind to the MHC complex on B cells, leading to rapid division and growth of B cells.
B cells then change into plasma cells, and secrete large quantities of finely formed antibodies specific to the foreign agent.
Some B cells are also converted into memory cells, allowing for a faster antibody-mediated immune response upon future infection.