Human Physiology ❮ 193The immune system is a two-tiered defense mechanism. It consists of nonspecific
immunityandspecific immunity.Nonspecific immunity is exactly how it sounds—it is
the nonspecific prevention of the entrance of invaders into the body. Saliva contains an
enzyme called lysozymethat can kill germs before they have a chance to take hold.
Lysozyme is also present in our tears, providing a nonspecific defense mechanism for our
eyes. The skin covering the entire body is a nonspecific defense mechanism—it acts as a
physical barrier to infection. The mucous lining of our trachea and lungs prevent bacteria
from entering cells and actually assists in the expulsion of bacteria by ushering the bacteria
up and out with a cough. Finally, remember how we told you that you did not want to get
out of the bus in the stomach? That is also the case for bacteria—it is a dangerous place for
them as well. The acidity of the stomach can wipe out a lot of potential invaders.
A nonspecific cellular defense mechanism is headed up by cells called phagocytes.
These cells, macrophagesandneutrophils,roam the body in search of bacteria and dead cells
to engulf and clear away. Some assistance is offered to their cause by a protein molecule
calledcomplement.This protein makes sure that molecules to be cleared have some sort
of identification displaying the need for phagocyte assistance. Complement coats these
cells, stimulating phagocytes to ingest them. Cells involved in mechanisms that need
cleanup assistance, such as platelets, have the ability to secrete chemicals that attract
macrophages and neutrophils to places such as infection sites to help in the elimination of
the foreign bacteria. They are nonspecific because they are not seeking out particular
garbage... they are just looking for something to eat.
A prime example of a nonspecific cellular response is inflammation. Let’s say that you
pick up a tiny splinter as you grab a piece of wood. Within our tissues lie cells known as
mast cells. These cells contain the signal histaminethat calls in the cavalry and initiates the
inflammation response. Entrance of the splinter damages these mast cells, causing them to
release histamine, which migrates through the tissue toward the bloodstream. Histamine
causes increased permeability and blood flow to the injured tissue. The splinter also causes
the release of signals that call in our nonspecific phagocytic cell friends, which come to the
site of the injury to clear away any debris or pathogens within the tissue. The redness and
warmth associated with inflammation occur because of the increase in blood flow to the
area that occurs in this process.
The immune system also contains defense mechanisms, which are quite specific. One
such defense mechanism involves a type of white blood cells called lymphocytes.There are
two main flavors of lymphocytes: B cells and T cells. These cells are made in the bone marrow
of the body and come from cells called stem cells.B cells mature in the bone marrow, and T
cells mature in the thymus. B cells can differentiate into plasma cells and memory B cells, and
the two main types of T cells are helper T cells and cytotoxic T cells. Cytotoxic T cells are
the main players involved in cell-mediated immunity. Helper T cells,which assist in the acti-
vation of B cells, recognize foreign antigens on the surface of phagocytic cells and bind to
thesecells. After binding, they multiply to produce a bunch of T cells that pump out chem-
ical signals, which bring in the B cells to respond.
We have arrived at the vaccination site in the left arm, and things are definitely heat-
ing up here. An antigenis a molecule that is foreign to our bodies and causes the
immune system to respond. What is occurring right now is the process called the
primary immune response.Every B cell has a specific (randomly generated) antigen
recognition site on its surface. B cells patrol the body looking for a particular invader.
When a B cell meets and attaches to the appropriate antigen, it becomes activated, and
the B cell undergoes mitosis and differentiation into the two types of cells mentioned
earlier:plasma cellsandmemory cells.The plasma cells are the factories that produceBIG IDEA 2.D.3
Organisms are
affected by disrup-
tions to their home-
ostasis (e.g.,
pathogens).
BIG IDEA 2.C.2
Organisms respond
to changes in their
environment (e.g.,
infections).
BIG IDEA 2.D.4
Animals have a
variety of chemical
defenses against
infections that affect
homeostasis.
BIG IDEA 3.D.2
Helper T cells
communicate with
other cells through
direct contact.
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