T lymphocytes are responsible for eliminating body cells that have become infected by pathogens (or that have become pathogenic themselves), unlike B cells that target the pathogen directly.
Aside from playing a different role in the immune system, T cells can be differentiated from B cells by the type of antigen receptor they carry on their cell surface.
T cell receptors are closely related to the immunoglobulins that form the B cell receptors, however, their structure and recognition mechanisms are very different. Both immunoglobulins and T cell receptors have a variable region that forms the antigen binding site, however T cell receptors are much smaller than antibodies and are not produced in a secretable form.
Naive T cells are those that have survived tolerance testing but that have not yet encountered the antigen that matches
Once a naive T cell recognizes a pathogen matching its receptor specificity for the first time, it can differentiate into a helper T cell. On its first antigen encounter, the naive T cell begins to proliferate, producing many daughter T helper cells possessing identical receptor specificity. Helper T cells play many key roles in activating other cells of the adaptive response. For example, helper T cell signals are required to initiate B cell differentiation and to activate cytotoxic T cells to kill infected cells.
This is another type of T cell that a naive T cell can differentiate into after recognizing an antigen for the first time. Cytotoxic T cells kill cells that are infected with pathogens and present the pathogenic antigen on their cell surface.
This T cell type acts to limit and control the response of other immune cells to avoid hyperstimulation of the immune system which could ultimately cause death.