Anything that causes an immune response is called an antigen. An antigen may be harmless, such as grass pollen, or harmful, such as the flu virus. Disease-causing antigens are called pathogens. The immune system is designed to protect the body from pathogens.
In humans, the immune system begins to develop in the embryo. The
immune system starts with hematopoietic (from Greek, "blood-making")
stem cells. These stem cells differentiate into the major players in the
immune system (granulocytes, monocytes, and lymphocytes). These stems
cells also differentiate into cells in the blood that are not involved
in immune function, such as erythrocytes (red blood cells) and
megakaryocytes (for blood clotting). Stem cells continue to be produced
and differentiate throughout your lifetime.
Hematopoietic stem cells produce cells in blood and lymph
By the time a baby is born, the immune system is a sophisticated
collection of tissues that includes the blood, lymphatic system, thymus,
spleen, skin, and mucosa. The immune system is typically divided into two categories--innate and
adaptive--although these distinctions are not mutually exclusive.
Innate immunity
Innate immunity refers to nonspecific defense mechanisms that come into
play immediately or within hours of an antigen's appearance in the body.
These mechanisms include physical barriers such as skin, chemicals in
the blood, and immune system cells that attack foreign cells in the
body. The innate immune response is activated by chemical properties of
the antigen.
Adaptive immunity
Adaptive immunity refers to antigen-specific immune response. The
adaptive immune response is more complex than the innate. The antigen
first must be processed and recognized. Once an antigen has been
recognized, the adaptive immune system creates an army of immune cells
specifically designed to attack that antigen. Adaptive immunity also
includes a "memory" that makes future responses against a specific
antigen more efficient.
The cellular system
- T-cells differentiate in the thymus, and have a specific receptor for a fragment of antigen..
- Cytotoxic T-cells contain a surface protein called CD8 and destroy pathogen infected cells, cancer cells, and foreign cells (transplanted organs).
- Helper T-cells contain a surface protein called CD4 and regulate both the cellular and humoral immune systems. This regulation reduces autoimmunity.
- Autoimmune disease- self immunity. Some examples include rheumatic fever, rheumatoid arthritis, ulcerative colitis, myasthenia gravis, etc.
Immunological response
The graph shows a very important feature of the immune response. When first exposed to antigen "A",
we begin to make low levels of antibody in about a week However, a second exposure to antigen "A"
produces a much faster response,
and several orders of magnitude higher levels of antibody. The ability of antibody to
bind antigen also increases dramatically in the secondary response. Injecting a new antigen "B" with "A"
shows that a memory or prior exposure is required for the accelerated response. The memory of
antigen and the stimulated response is the basis for success in vaccination programs.
The Clonal Selection Theory
- The immune systems produces Billions of kinds of B-cells each making one kind of antibody receptor.
- The presence of antigen leads to the proliferation and differentiation of clones that have antibody capable of binding the antigen. In the diagram the "green" antigen binds to the green antibody on a B-cell. The color code means that only this antibody receptor on the cell binds free antigen.
- The "green" helper T-cell must give a stimulatory signal to allow a particular B-cell to be selected. This step allows a regulation or control of the process.
- The antigen driven selection produces memory cells and plasma cells secreting antibody capable of binding the original selecting antigen with high affinity..
- If antigen appears in the organism a second time, then the memory cells are already present at high levels, and produce a more rapid and much stronger immune response.
By
Aparna S M
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