Question

Indicate from how T-cell receptors differ from B cell receptors in the way that they recognize antigen. Your answer must include: peptides, antigen-presenting cells, MHC molecules, and antigen-binding sites

Answer 1

T cell receptors are different from B cell receptors in the way that they recognise an antigen, because unlike B cells which possess membrane bound immunoglobulin molecules on their surface which recognise the antigen in it's native form, T cell receptors can only identify peptide segments of foreign protein bound to an MHC on a cell surface and bind to it.

The immunoglobulin molecules that act as the B cell receptor of the same antigen specificity are then released into the bloodstream as antibodies against the specific antigen. These molecules generally have two variable region and these variable domains on the chain act as the antigen binding site. The antigen receptors of T cells however, function only as membrane bound proteins.

T cell receptors are similar in structure to the B cell receptors with variable and constant domains. But unlike B cell receptor which has two, T cell receptors have only one antigen binding site.

As T cells cannot recognise the antigens in their native form, they respond to the antigen fragments present on the surface of the antigen presenting cells (APCs).

The antigen presenting cells are macrophages, dendritic cells and B lymphocytes. These cells ingest the antigens and then display the short peptide sequences of the fragments on the surface with the help of the surface molecules known as Major Histocompatibility Complex (MHC) which possess a peptide binding groove. MHC class I molecules are found in most body cells. MHC class II molecules are mostly found on B cells and macrophages.