WORLD OF MICROBIOLOGY AND IMMUNOLOGY Major histocompatibility complex (MHC)361•
2000 revealed the presence of magnetite crystals, which on
Earth are produced only in magnetotactic bacteria. The mag-
netite crystals found in the meteorite are identical in shape,
size and composition to those produced in Magnetospirillum
magnetotacticum. Thus, magnetite is a “biomarker,” indicat-
ing that life may have existed on Mars in the form of magne-
totactic bacteria. The rationale for the use of magnetotaxis in
Martian bacteria is still a point of controversy. The Martian
atmosphere is essentially oxygen-free and the magnetic field
is nearly one thousand times weaker than on Earth.
Magnetotactic bacteria are also of scientific and indus-
trial interest because of the quality of their magnets. Bacterial
magnets are much better in performance than magnets of com-
parable size that are produced by humans. Substitution of
man-made micro-magnets with those from magnetotactic bac-
teria could be both feasible and useful.See alsoBacterial movementMAJOR HISTOCOMPATIBILITY COMPLEX
(MHC)Major histocompatibility complex (MHC)
In humans, the proteins coded by the genes of the major his-
tocompatibilitycomplex (MHC) include human leukocyte
antigens (HLA), as well as other proteins. HLA proteins are
present on the surface of most of the body’s cells and are
important in helping the immune systemdistinguish “self”
from “non-self” molecules, cells, and other objects.
The function and importance of MHC is best under-
stood in the context of a basic understanding of the function of
the immune system. The immune system is responsible for
distinguishing foreign proteins and other antigens, primarily
with the goal of eliminating foreign organisms and other
invaders that can result in disease. There are several levels of
defense characterized by the various stages and types of
immune response.
Present on chromosome 6, the major histocompatibility
complex consists of more than 70 genes, classified into class
I, II, and III MHC. There are multiple alleles, or forms, of each
HLAgene. These alleles are expressed as proteins on the sur-
face of various cells in a co-dominant manner. This diversity
is important in maintaining an effective system of specific
immunity. Altogether, the MHC genes span a region that is
four million base pairs in length. Although this is a large
region, 99% of the time these closely linked genes are trans-
mitted to the next generation as a unit of MHC alleles on each
chromosome 6. This unit is called a haplotype.
Class I MHC genes include HLA-A, HLA-B, and HLA-
C. Class I MHC are expressed on the surface of almost all
cells. They are important for displaying antigenfrom viruses
or parasitesto killer T-cells in cellular immunity. Class I MHC
is also particularly important in organ and tissue rejection fol-
lowing transplantation. In addition to the portion of class I
MHC coded by the genes on chromosome 6, each class I MHC
protein also contains a small, non-variable protein component
called beta 2-microglobulin coded by a gene on chromosome- Class I HLA genes are highly polymorphic, meaning there
are multiple forms, or alleles, of each gene. There are at least
57 HLA-A alleles, 111 HLA-B alleles, and 34 HLA-C alleles.
Class II MHC genes include HLA-DP, HLA-DQ, and
HLA-DR. Class II MHC are particularly important in humoral
immunity. They present foreign antigen to helper T-cells,
which stimulate B-cells to elicit an antibodyresponse. Class II
MHC is only present on antigen presenting cells, including
phagocytes and B-cells. Like Class I MHC, there are hundreds
of alleles that make up the class II HLA gene pool.
Class III MHC genes include the complementsystem
(i.e. C2, C4a, C4b, Bf). Complement proteins help to activate
and maintain the inflammatory process of an immune response.
When a foreign organism enters the body, it is encoun-
tered by the components of the body’s natural immunity.
Natural immunity is the non-specific first-line of defense car-
ried out by phagocytes, natural killer cells, and components of
the complement system. Phagocytes are specialized white
blood cells that are capable of engulfing and killing an organ-
ism. Natural killer cells are also specialized white blood cells
that respond to cancer cells and certain viral infections. The
complement system is a group of proteins called the class III
MHC that attack antigens. Antigens consist of any molecule
capable of triggering an immune response. Although this list is
not exhaustive, antigens can be derived from toxins, protein,
carbohydrates, DNA, or other molecules from viruses, bacte-
ria, cellular parasites, or cancer cells.
The natural immune response will hold an infection at
bay as the next line of defense mobilizes through acquired, or
specific, immunity. This specialized type of immunity is usu-
ally what is needed to eliminate an infection and is dependent
on the role of the proteins of the major histocompatibility
complex. There are two types of acquired immunity. Humoral
immunity is important in fighting infections outside the body’s
cells, such as those caused by bacteria and certain viruses.
Other types of virusesand parasites that invade the cells are
better fought by cellular immunity. The major players in
acquired immunity are the antigen-presenting cells (APCs), B-
cells, their secreted antibodies, and the T-cells. Their functions
are described in detail below.
In humoral immunity, antigen-presenting cells, includ-
ing some B-cells, engulf and break down foreign organisms.
Antigens from these foreign organisms are then brought to the
outside surface of the antigen-presenting cells and presented
in conjunction with class II MHC proteins. The helper T-cells
recognize the antigen presented in this way and release
cytokines, proteins that signal B-cells to take further action. B-
cells are specialized white blood cells that mature in the bone
marrow. Through the process of maturation, each B-cell devel-
ops the ability to recognize and respond to a specific antigen.
Helper T-cells aid in stimulating the few B-cells that can rec-
ognize a particular foreign antigen. B-cells that are stimulated
in this way develop into plasma cells, which secrete antibod-
ies specific to the recognized antigen. Antibodies are proteins
that are present in the circulation, as well as being bound to the
surface of B-cells. They can destroy the foreign organism from
which the antigen came. Destruction occurs either directly, or
by tagging the organism, which will then be more easily rec-
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