194 ❯ STEP 4. Review the Knowledge You Need to Score Highantibodies that function in the elimination of any cell containing on its surface the anti-
gen that it has been summoned to kill. These antibodies, when released, bind to the
antigens, immobilizing them and marking them for the macrophages to engulf and elim-
inate. This type of immune response falls under the category of humoral immunity—
immunity involving antibodies.
Someone had a question? How do antibodies recognize the antigen they are designed
for? Excellent question. Antibodies are protein molecules with two functional regions.
One end is called the fragment antigen binding regionor Fab—this is what allows an anti-
body to recognize a specific antigen. It is designed by the plasma cell to have an Fabthat
binds to the antigen of interest. The other end, which binds to effector cells, is called the
Fcregion. There are five types of Fcregions, one for each of the five types of antibodies:
IgA, IgD, IgE, IgM, and IgG. Each antibody type serves a slightly different function and
is present in different areas of the body. When the antibodies bind to an antigen, comple-
ment gets involved, and this combination of antibodies and complement leads to the elim-
ination of the invader.
We see a hand raised in the back. Yes, you are correct that we neglected to mention the
memory cells. Very good. Memory cells contain the basis for the body’s secondary immune
responseto invaders. Memory cells are stored instructions on how to handle a particular
invader. When an invader returns to our body, the memory cells recognize it, produce anti-
bodies in rapid succession, and eliminate the invader very quickly. The secondary immune
response is much more efficient than the primary response. This is why few people are
infected by sicknesses such as chicken pox after they have had them once already—their
memory cells protect them. One important fact that does come up on the exam is that the
secondary immune response produces a muchlarger concentration of antibodies than does
the primary response.
Well, this is too good to be true... we just got word that this body was just recently
infected by a virus. This will allow us to look at the other side of the immune response:
cell-mediated immunity.This type of immunity involves directcellular response to inva-
sion as opposed to antibody-based defense. The virus that infected this poor sap made it
past the humoral immunity system because it entered into the host’s cells. This brings the
cytotoxic T cells into play. The cells infected by the virus are forced to produce viral anti-
gens, some of which show up on the surface of the cell. The cytotoxic T cells recognize
these cells and wipe them out.
You might wonder how these T cells avoid killing allcells. All the cells of the body,
except for red blood cells, have on their surface antigens called class I histocompatibility
antigens(major histocompatibility complex [MHC]). The MHC I antigens for each person
are slightly different, and the immune system accepts as friendly any cell that has the iden-
tical match for this antigen. Anything with a different MHC is foreign. This is the reason
that organ donation often fails—the donor and the recipient have incompatible MHCs.
There are also class II histocompatibility antigens,which are found on the surface of the
immune cells of the body. These antigens play a role in the interaction between the cells of
the immune system.
Well, we’d like to thank you for joining us on our tour of the body. We’ve seen a lot
of things today and—whoa! We’ve been hit by something—and hit hard! Oh, dear....
Folks, we don’t want you to be alarmed, but it appears that our rival tour company has
played a bit of a practical joke on us. Apparently as we were observing the B cell’s interac-
tion with the vaccine, they attached a series of antigens and complement proteins to the
surface of our bus. That loud noise you just heard was the sound of a macrophage taking
us in... oh, dear, this is bad. Oh, no... folks, brace yourselves! The macrophage is about
to ——— (transmission ended).BIG IDEA 4.C.1
Molecular varia-
tions in Fabenable
cells to recognize a
wider range of
antigens.