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The Immune System

Cytotoxic T cells (Tc cells) destroy virally infected cells and tumor cells, and are also implicated in
transplant rejection. These cells are also known as CD8+ T cells, since they express the CD8
glycoprotein at their surface.

Helper T cells, (Th cells) are the "middlemen" of the adaptive immune system. Once activated,
they divide rapidly and secrete small proteins called cytokines that regulate or "help" the immune
response. These cells (also called CD4+ T cells) are a target of HIV infection; the virus infects the cell
by using the CD4 protein to gain entry. The loss of Th cells as a result of HIV infection leads to the
symptoms of AIDS.

Memory T cells are a subset of antigen-specific T cells that persist long-term after an infection has
resolved. They quickly expand to large numbers of effector T cells upon re-exposure to their cognate
antigen, thus providing the immune system with "memory" against past infections. Memory cells may
be either CD4+ or CD8+.

Regulatory T cells (Treg cells), formerly known as suppressor T cells, are crucial for the
maintenance of immunological tolerance. Their major role is to shut down T cell mediated immunity
towards the end of an immune reaction and to suppress auto-reactive T cells that escaped the process of
negative selection in the thymus. Two major classes of regulatory T cells have been described,
including the naturally occurring Treg cells and the adaptive Treg cells.

Treg cells (also known as CD4+CD25+FoxP3+ Treg cells) arise in the thymus, whereas the
adaptive Treg cells (also known as Tr1 cells or Th3 cells) may originate during a normal immune
response. Naturally occurring Treg cells can be distinguished from other T cells by the presence of an
intracellular molecule called FoxP3. Mutations of the FOXP3 gene can prevent regulatory T cell
development, causing the fatal autoimmune disease IPEX.

'Natural Killer T cells''' (NKT cells) are a special kind of lymphocyte that bridges the adaptive
immune system with the innate immune system. Unlike conventional T cells that recognize peptide
antigen presented by major histocompatibility complex (MHC) molecules, NKT cells recognize
glycolipid antigen presented by a molecule called CD1d. Once activated, these cells can perform
functions ascribed to both Th and Tc cells (i.e. cytokine production and release of cytolytic/cell killing
molecules).

THE FUNCTIONS OF T LYMPHOCYTES T lymphocytes cells help with all components of
the immune system, including cell elimination by killer T cells and maintaining roles by helper and
suppressor T cells. Although the specific mechanisms of activation vary slightly between different
types of T cells, the "two-signal model" in CD4+ T cells holds true for most.

The Immune System Pioneers

Ilya Mechnikov and the Phagocyte Cells

In 1882, a Russian scientist named Ilya Mechnikov was experimenting with the larvae of the sea
star. He stuck a thorn in the larvae and then he noticed that something really weird happened. Strange
cells started gathering near the point of insertion. The cells that were surrounding the thorn were eating
any foreign substance that was entering through the ruptured skin. Mechnikov decided to name these

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