Whereas B cells all have the same goal as each other, to secrete antibodies, T cells are much more varied and complex. There are two major groups of T cells: Helper (CD4) T cells and Cytotoxic (CD8) T cells. Also, when CD4 T cells become activated, they further differentiate into subgroups such as T helper 1 cells, T helper 2 cells, and others, which will be covered in subsequent sections.
T cells recognize and respond to antigens quite differently than do B cells. B cell receptors recognize the antigen's molecular structures; generally, the antigen fits into the receptor. T lymphs, on the other hand, recognize short peptide sequences, which are much smaller and not folded into any structure. The antigen must first be "processed" to produce these peptide sequences.
The T cell receptor is also a member of the immunoglobulin superfamily (see image on page 15, B lymphocyte receptors). There are other similarities to the immunoglobulin (B cell) receptor, such as variable and constant domains. There are many differences, however. B cells have five classes of immunoglobulin, but there are only two classes of the T cell receptor, αβ receptors and γδ receptors. The vast majority of T cells have αβ. The γδ cells are associated mainly with the intestinal mucosa and lack the specificity of most T cells.
One of the most fundamental differences between the two types of lymph is that the processed peptide sequence must be "presented" to the T cell by a type of molecule called "MHC I or MHC II." Another co-receptor, either CD4 (on T helper cells) or CD8 (on T cytotoxic cells), must be present on the T cell to recognize that MHC molecule.
The image on the top right depicts a T cell αβ receptor associated with a CD4 molecule. The T cell receptor will attach to an antigen presented by MHC II, and CD4 will recognize the MHC II molecule. Note that three additional molecules are associated with the T cell receptor, collectively called CD3. CD3 sends the intracellular signal, much like the alpha-beta B cell coreceptor sends the intracellular signal in B cells.
The image on the bottom right shows antigen presented with the MHC I molecule to a CD8 T cell. CD8 is attached to the MHC portion, and the presented antigen attaches to the T cell receptor portion. CD3 is not shown in this image, but it is present.
