What molecule on the surface of most T cells recognizes antigen?

The molecule on the surface of most T cells that recognizes antigen is the T cell receptor (TcR), which is composed of two chains, typically referred to as alpha and beta. The TcR is essential for the immune response as it specifically binds to antigens presented by major histocompatibility complex (MHC) molecules on the surface of antigen-presenting cells.

When a T cell encounters an antigen that matches its receptor, it triggers a signaling cascade that activates the T cell, leading to a variety of immune responses. This specificity is crucial, as it allows T cells to differentiate between self and non-self antigens, ensuring an appropriate immune response.

In contrast, the other options do not serve the primary role of antigen recognition in T cells. IgT is not a recognized isotype in immune responses; MHC proteins are involved in presenting the antigen to T cells but do not directly recognize it themselves; while CD3 is part of the T cell receptor complex that aids in signaling upon antigen recognition but is not the molecule that directly binds the antigen. Thus, the T cell receptor's role as the antigen recognition molecule is foundational in understanding T cell functionality in the immune system.