The thymus trains immature T cells from the bone marrow to become mature cells that only attack foreign cells. In the thymus cortex, T cells encounter epithelial cells presenting various antigens. T cells that respond to foreign antigens are selected to survive and migrate to the medulla, while others undergo apoptosis. This is called positive selection.

2. THYMUS
The function of the thymus is to receive immature T cells that are
produced in the red bone marrow and train them into functional,
mature T cells that attack only foreign cells. T cells first reside within
the cortex of the thymus where they come in contact with epithelial
cells presenting various antigens. The immature T cells that respond
to the antigens corresponding to foreign cells are selected to survive,
mature, and migrate to the medulla while the rest die via apoptosis
and are cleaned up by macrophages. This process is known as
positive selection.

3. BONE MARROW
The yellow tissue in the center of the bones produces
white blood cells. This spongy tissue inside some
bones, such as the hip and thigh bones, contains
immature cells, called stem cells, according to the NIH.
Stem cells, especially embryonic stem cells, which are
derived from eggs fertilized in vitro (outside of the
body), are prized for their flexibility in being able to
morph into any human cell.

4. T AND B CELLS
T- and B-cells are highly specialized defender cells -
different groups of cells are tailored to different germs.
When your body is infected with a particular germ, only
the T- and B-cells that recognize it will respond. These
selected cells then quickly multiply, creating an army of
identical cells to fight the infection. Special types of T-
and B-cells 'remember' the invader, making you immune
to a second attack.

5. ANTIBODY AND ANTIGEN
When an antigen enters the body, the immune system produces
antibodies against it. Antibodies are always Y-shaped. It is like a
battle with the army (antibody) fighting off the invader (antigen). A
type of white blood cell called a lymphocyte recognizes the
antigen as being foreign and produces antibodies that are specific
to that antigen. Each antibody has a unique binding site shape
which locks onto the specific shape of the antigen. The antibodies
destroy the antigen (pathogen) which is then engulfed and
digested by macrophages.