First it is important to understand how our immune response works. All specific immune responses start with the recognition of dangerous matter that can cause disease; think of it as bacteria, or even cancer cells. Considering our white blood cells that form most of our immune defense, dendritic cells are particularly capable at forming the bridge between the recognition of danger and mobilizing protective defense. On meeting dangerous matter, dendritic cells take bits and pieces thereof, and migrate to the nearest lymph node. Inside dendritic cells these foreign proteins are diced into small fragments (peptides) and loaded onto cell proteins, called class I or class II MHC molecules. These MHC molecules can be understood as serving trays that bring the peptide fragments back to the cell surface and present them to T cells.
When the dendritic cells arrive in the lymph node, they encounter T cells. T cells are another type of white blood cell, specialized in fighting the encountered danger in the body. The dendritic cell presents its antigen material to the T cells. One set of the T-cells can only detect danger fragments when displayed by MHC I molecules, whereas another does this for fragments displayed by MHC II molecules. The T cells are like bloodhounds that can multiply: once they recognize the load presented by the MHC serving tray, they start dividing to form an army of cells and forcefully attack the dangerous felon. After the danger is removed, the immune response winds down and balance is restored. In many cases, we as people do not even notice these encounters as they occur inside our bodies.