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The Immune System

accumulates a mass called a seroma may form. Treatment of a seroma may involve the removal of the
fluid with a needle into a syringe, a process called aspiration.

Phagocytosis by neutrophils and macrophages

In the event of a break in the skin, neutrophils, monocytes (and macrophages) arrive and attempt to
engulf and destroy the invaders. Phagocytosis is receptor-mediated event, which ensures that only
unwanted particles are ingested. Stimulated macrophages can bring about an explosive increase in the
number of leukocytes by producing Colony Stimulating Factors (CSFs). The CSFs pass by way of the
blood to the bone marrow, where they stimulate the production and the release of white blood cells
(WBCs), primarily neutrophils. Lymphocytes in nearby lymph nodes produce specific antibodies to
attack the microbes. During the conflict, some neutrophils die and become mixed with dead tissue,
bacteria, living white cells, etc. This thick yellow-white fluid is called pus. When a person has an
illness, an examination of the numbers and types of WBC's in their blood can be very useful.

Complement System

The complement system is a biochemical cascade of the immune system that helps clear pathogens
from an organism, and promote healing. It is derived from many small plasma proteins that work
together to form the primary end result of cytolysis by disrupting the target cell's plasma membrane.

Complement is activated by antigen-antibody complexes and causes holes to form in the plasma
membrane of foreign microbes or cells (lysis). The complement system is considered a nonspecific
defense, but it can be activated against specific microbes that have been marked with antibodies.
Hemolytic transfusion reactions are caused by complement activation after a person expresses
antibodies against the antigens found on the inappropriately donated blood. Hemolytic Disease of the
Newborn (HDN) is due to maternal antibodies against the Rh factor crossing the placenta, binding to
the baby's red blood cells, and stimulating the baby's own complement system to lyse its red blood
cells.

Interferon in response to viral infection

Interferon (IFNs) are naturally occurring glycoproteins involved in non-specific immune
responses. Interferons do just as their name states they "interfere" with viral growth. Interferons are
initiated from a cell that has been infected by a virus. When a cell has been infected by a virus the virus
will then cause the cell to make viral nucleic acid. This nucleic acid acts as a signal and it causes the
cell to realize that it has been infected with a virus. So the cell will start making and sending out
interferons. The IFN's that the cell sends out go to nearby healthy cells and warns them of a virus. The
healthy cells then start intracellular changes that help the cells to be more resistant to the virus.

Adaptive Defense (Specific Defense--third line of defense)

This part of the immune system directly targets invading microbes. Our specific immune defenses
respond to antigens. An antigen is a protein (or polysaccharide) molecule, typically on the cell
membrane, that the body recognizes as nonself. They are found on microbes, foreign cells, or on cancer
cells. Normally our immune system does not respond to our own antigens (if it does, then this is an

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