MHC class II is necessary for antigen recognition by CD4-positive T cells.
MHC is the abbreviated form for Major Histocompatibility Cells. When the antigen enters the body, it is taken by Antigen Presenting Cell (APC). The APCs are dendritic cells, B cells and macrophages.
The APCs process the antigen and present it on its surface, in bonded form with MHC. Antigen presented by MHC class II forms complex with CD4⁺ T cell. Similarly, antigen presented by MHC class I forms complex with CD8⁺ T cell.
The antigens are further activated upon by respective T cells. The CD4⁺ T cells are called helper cells and CD8⁺ T cells are called cytotoxic cells. Former increase the immune response while latter directly kills the cell.
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