Thymus
The thymus gland lies in the upper part of the mediastinum behind
the sternum and extends upwards into the root of the neck. It weighs
about 10 to 15 g.(about half an ounce) at birth and begins to grow
until the individual reaches puberty when it begins to atrophy.
It's
maximum weight is around 30 - 40g (around 1 to 1.5 ounces) by the
age of 40 it has returned to it's weight at birth. The thymus
consists of two lobes connected by areolar tissue. The lobes are
enclosed in a fibrous capsule which dips into their substance
dividing them into lobules that consist of an irregular branching
framework of epithelial cells and lymphocytes.

Function
Lymphocytes originate from haemocytoblasts (stem cells) in red bone
marrow. Those that enter the thymus mature and develop into
activated T-lymphocytes i.e. able to respond to antigens encountered
elsewhere in the body.

They then divide into two groups :
those that enter the blood, some of which remain in
circulation and some lodge in other lymphoid tissue
those that remain in the thymus gland and are the source of
future generations of T-lymphocytes.

The maturation of the thymus and other lymphoid tissue is stimulated
by thymosin, a hormone secreted by the epithelial cells that form
the framework of the thymus gland. Involution of the gland begins in
adolescence and, with increasing age the effectiveness of T-
lymphocyte response to antigens declines.


T Cells
T cells have two major roles in immune defense. Regulatory T cells
are essential for orchestrating the response of an elaborate system
of different types of immune cells.
Helper T cells, for example, also known as CD4 positive T cells
(CD4+ T cells), alert B cells to start making antibodies; they also
can activate other T cells and immune system scavenger cells called
macrophages and influence which type of antibody is produced.
Certain T cells, called CD8 positive T cells (CD8+ T cells), can
become killer cells that attack and destroy infected cells. The
killer T cells are also called cytotoxic T cells or CTLs (cytotoxic
lymphocytes).


Certain T cells, which also patrol the blood and lymph for foreign
invaders, can do more than mark the antigens; they attack and
destroy diseased cells they recognize as foreign. T lymphocytes are
responsible for cell-mediated immunity (or cellular immunity). T
cells also orchestrate, regulate and coordinate the overall immune
response. T cells depend on unique cell surface molecules called the
major histocompatibility complex (MHC) to help them recognize
antigen fragments.



T lymphocytes become CD4+ or helper T cells, or they can become CD8+
cells, which in turn can become killer T cells, also called
cytotoxic T cells.



Immune system process
Activation of helper T cells
After it engulfs and processes an antigen, the macrophage displays
the antigen fragments combined with a Class II MHC protein on the
macrophage cell surface. The antigen-protein combination attracts a
helper T cell, and promotes its activation.

Activation of cytotoxic T cells
After a macrophage engulfs and processes an antigen, the macrophage
displays the antigen fragments combined with a Class I MHC protein
on the macrophage cell surface. A receptor on a circulating, resting
cytotoxic T cell recognizes the antigen-protein complex and binds to
it. The binding process and a helper T cell activate the cytotoxic T
cell so that it can attack and destroy the diseased cell.

Activation of B cells to make antibody

A B cell uses one of its receptors to bind to its matching antigen,
which the B cell engulfs and processes. The B cell then displays a
piece of the antigen, bound to a Class II MHC protein, on the cell
surface. This whole complex then binds to an activated helper T
cell. This binding process stimulates the transformation of the B
cell into an antibody-secreting plasma cell.


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