T
cells (and B cells) are part of the adaptive immune response that
takes 4-7 days after infection to become effective. T cells are lymphocytes
or cells of the lymphatic system. T cells are so called because they
develop in the thymus, an organ located in the upper chest just above
the heart. T cells are of two types: helper T cells (which have a
marker on their cell surface called CD4) and killer T cells (which
have the CD8 marker on their surface). Killer T cells (also called
cytotoxic T lymphocytes or CTLs) directly attack body cells that are
infected with a virus or malignant or abnormal tumor cells, cells
that antibodies would never perceive.
Helper T cells are the "generals" of the immune system,
calling into play and directing the activity of killer T cells. The
helper T cells are essential for an effective immune response since
they activate other immune cells including most B cells to produce
antibody. Individual T cells are targeted against the specific antigen
signatures of viruses and bacteria, and when helper T cells encounter
their specific antigens, they become activated and quickly expand
in number. In AIDS, the virus HIV attacks activated T helper cells,
it kills off the very cells that should coordinate the body's antiviral
defenses, and as a result people with HIV usually have few or no HIV-specific
T helper cells!
How Do T Cells See
Their Antigen?
T cells have almost the same diversity as B cells that allow them
to recognize various different pathogens. However, T cells recognize
a part of the antigen presented on infected cells or cancerous cells
or transplanted organ cells (immune rejection). It is absolutely fascinating
how T cells know which cells are 'normal' versus 'infected' or 'abnormal'
or 'non-self'. T cells have receptors on their surfaces that recognize
HLA (human leukocyte antigen) markers that are present on all body
cells. These are the markers (along with blood group markers) that
are used to determine whether a donor transplanted organ or bone marrow
will be accepted or rejected by the recipient. The HLA molecules on
the cell surface bind and present protein fragments from within the
cell. Thus, they allow the T cells to sample the various proteins
within the cell and get activated only if they recognize a 'foreign-looking'
protein fragment bound to the HLA molecule on the cell surface.
How Do Helper T Cells
'Help' And Killer T Cells 'Kill'?
T cells act by releasing certain chemicals known as cytokines. Cytokines
are proteins (secreted by other immune cells too) that are responsible
for activating cells, triggering them to grow and divide or to die.
A complex network of cytokines is secreted by the helper T cells that
determine the course of the immune response. Killer T cells, on the
other hand, release cell-damaging enzymes that create holes in the
membranes of the target cells and trigger them to undergo programmed
cell death.
Cytokines, are now being used-alone or in combination to treat various
disorders such as cancer, rheumatoid arthritis and AIDS among others.
The aim of these treatments is to mimic part of the immune response
that fools the cells to either get activated and divide such as in
the case of the immune deficiency disease AIDS or to slow down a destructive
immune response such as in the case of the autoimmune disease rheumatoid
arthritis.
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