Immune system consists of many different cells, some of which acts as a part of innate immunity while lymphocytes are major players.
Innate Immunity is present right from the birth but acquired or adaptive immunity is acquired during the course of life.
Primary immune response to Ag occurs on the first occasion. Subsequent encounter of B and T cells with same Ag leads to secondary response.
MHC molecules bind peptide fragments derived from pathogens and display them on the cell surface for recognition by the appropriate T cells.