Whereas B cells all have much 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 molecular structures of the antigen, and generally speaking, the antigen fits into the receptor. T lymphs, on the other hand, recognize short peptide sequences, which are much smaller and are not folded into any structure. Additionally, the antigen much first be "processed" to produce these peptide sequences.
The T Cell receptor itself 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 lymphs 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 CD 8 (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 in association with a CD 4 molecule. The T Cell receptor will attach to an antigen presented by MHC II, and CD 4 will recognize the MHC II molecule. Note that there are three additional molecules associated with the T Cell receptor, collectively known as CD 3. CD 3 is responsible for sending 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 being 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 actually present as well.
