Clonal expansion of antigenspecific T cells

During the primary immune response, antigen-spe-cific T cells are clonally expanded. It is believed that this expansion provides a further level of protection from reinfection. The mechanisms involved in the development and maintenance of T cell memory are still unclear. After the initial T cell expansion, the antigen-specific T cells are downregulated through induction of programmed cell death and anergy. This stage is followed by the development of an antigen-specific memory T cell population. This population of T cells can remain with the individual for a lifetime, and can provide protection from further infections by the organism that expresses the antigen. There are two proposed mechanisms for how the body maintains these antigen-specific memory T cells. The first is that once these memory cells develop, they are extremely long-lived and can survive indefinitely to provide protection for the individual. The second hypothesis is that a small but significant amount of antigen persists in the individual. This antigen-persistence provides a continual level of stimulation for these memory T cells and is able to stimulate the production of new memory T cells.

Antigens that do not induce acquired immunity

Interestingly, not all antigens induce immunological memory. The best studied of these are T-independent (TI) antigens. TI antigens are capable of stimulating B cell proliferation and differentiation in the absence of cognate T cell help. Common TI antigens are polysaccharides and polymers, both of which are composed of highly repetitive subunits. The reason that these antigens are unable to effect an acquired immune response is still unclear. However, it is known that TI antigens tend to stimulate a subset of B cells, CD5+ cells, that may not be capable of differentiating into memory cells.

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