T helper cells recognizes -
MHC restriction

T cells (in contrast to B cells) cannot be activated by soluble antigens.
Therefore, presentation of processed antigen by antigen presenting cells is required for induction of cell mediated immunity.
T cell receptors recognizes peptide antigens that are displayed by major histo compatibility complex (MHC) molecules on the surface of antigen presenting cells.
T cells develop MHC restriction so that :-


Helper (CD4) T cells respond only to foreign antigens presented along with HLA class II (MHC class II).
CD8 T cells respond to antigens presented along with HLA class I (MHC class I).
MHC restriction refers to the fact that a given T cell will recognize a peptide antigen only when it is bound to a particular MHC molecule.
MHC class I
MHC class II
Processed peptides
Surface Ig