Understanding the Role of MHC Molecules in Immunity and the Importance of Vitamin D

MHC molecules and immunity
The cell-mediated adaptive immunity system is basically regulated by MHC, which is
a Major Histo-compatibility Complex molecule (Giles et al. 2015). The immune system of
the body fights the pathogens that succeed in invading the normal tissues. Immune system
protects the human body in various ways against bacteria, viruses, parasites, cancerous cell or
any other. Pathogens enter into the body in an invasive way. Two types of immune system
exist, innate immunity and adaptive immunity. Innate immune defences are mediated by
white blood cells or WBC, such as granulocytes, monocytes/macrophages and natural killer
cells, and by antibacterial proteins, such as acute phase and complement proteins, circulating
in blood.But, in case of adaptive immunity, some specific defences against an invader are
developed (Rock, Reits and Neefjes 2016). However, several forms of adaptive immunity are
there and they are humoral and cell mediated. In order to destroy the antigens, antibodies
appear in the body fluids in case of humoral adaptive immunity system but, in cell mediated
immunity system cells can destroy other cells become active. They destroy all the disease
infected cells (Rock, Reits and Neefjes 2016).
Function of MFC
Some specific functions of MHC molecules have been found by the researchers.
MHC molecules helps to introduce T-cell antigen receptors (and, in parallel, B-cell antigen
receptors for B cells) at an early stage. Inside the human body cells, proteins are broken down
into short fragments (Giles et al. 2015). Those short fragmented proteins can be displayed as
peptide antigens by MHC molecules. The self peptide, derived from the own proteins, as well
as the foreign peptides derived from the invading pathogens is displayed by MHC molecules
(Rock, Reits and Neefjes 2016). HLA (human leukocyte antigen) present in the MHC of
human body also helps in monitoring the amount of MHC-presented antigens that destroys

cancerous cells which displays increased amount of self-antigens (Moutsianas et al. 2015).
There are a vast number of potential peptide targets but the number of MHC proteins is
limited and due to this purpose MHC proteins are highly effective in binding several types of
peptides. Moreover, MHC proteins have the capability of binding peptides of different kinds
and even of different structures. Due to this unique property, MHC proteins are different from
other proteins or molecules. Tissue allorecognition is another function of MHC and it plays a
major role in preventing successful transplantation of organ (Moutsianas et al. 2015).
MHC and antigen presentation
MHC is useful in controlling the process in which the immune system of human body
detects as well as responds to some specific antigens. The MHC molecules also control the
antigen specificity of T-cell recognition (Giles et al. 2015). There are two different classes of
MHC molecules, class I as well as class II. Both the classes have similarity in function of
involving the delivery of very short peptides into the surface of cell recognition and it
basically takes place by CD8+ and CD4+ T cells respectively. It is possible to stimulate some
specific T-cells by MHC class I which is basically located on all cells which are nucleated
(Cho et al. 2015).
Difference between MHC class I and MHC class II
MHC is known to be highly polymorphic and in the immune function, the role of it
significant. In E. jankowskii, low level of MHC polymorphism was revealed and it was
similar to that in E.Cioides. There is a difference between the two classes of MHC. The
properties are not similar. The class I is the glycoproteins, which are expressed upon the
surface of all the nucleated cells. The main role of the class I MHC is the presentations of
peptide antigens into the TC (cytotoxic T) cells. The molecules of MHC class I consists of
one membrane spanning alpha chain which is encoded by MHC gene and one beta chain,

which is encoded by the beta2 microglobulin gene (Cho et al. 2015). It also presents foreign
intracellular antigens. But, in case of MHC class II molecules, it consists of two membrane
spanning chains, alpha and beta, but their sizes are similar and both are produced by the
MHC genes. The glycoproteins of MHC class II is present only on some specialized antigen
presenting cells. It also presents 14-18 amino acid peptides which is greater than MHC I. The
class II of MHC also presents foreign extracellular antigen that induces antibody production
as well. The inflammatory response increases the blood flow to the inflammatory area and it
brings immune cells to the site (Van der Meijden et al. 2016). All these properties distinguish
MHC class II from the properties of the MHC class I. Again, MHC class II is basically is a
class of major histo-compatibility complex molecules. These are generally found only on
antigen-presenting cells that includes mononeuclear phagocytes, dendritic cells, etc. All the
cells are extremely important in initiating the immune responses. MHC, the group of genes is
also useful in encoding the proteins found on the surface of cells, and it helps in the
recognition of antigens. At the same time it also determines the histo-compatibility (Giles et
al. 2015). MHC molecule is generally found in human chromosome and can be termed as
human leukocyte antigen (HLA).
MHC and Autoimmunity
Some molecules of MHC are directly associated with inflammatory diseases as well
as with the increased risk of autoimmune. In the year 1967, it was first found that MHC
HLA-B antigens increased frequency among the patients having Hodgkin’s lymphoma. Apart
from that, multiple sclerosis, rheumatoid arthritis, Crohn’s disease and some other health
conditions of human being are also associated with some specific MHC molecules (Hauser et
al. 2017). In an analysis, conducted by the association of MHC disease revealed that a
susceptibility of shared disease is there to the alleles that arise from HLA-DR4 haplotypes.
Simultaneously, the analysis indicates that there is a common as well as disease specific

association between autoimmunity and the MHC. The exact and specific mechanism behind
the autoimmunity and MHC molecules has not properly been found in the researches but it
potentially reflects a breakdown in tolerance to self-antigens in the antigen presentation of
normal MHC class II. Therefore, some specific class II alleles work as the determinants of
auto-antigen targeting (Cho et al. 2015).
MHC and tissue allorecognition
Transplant rejection
Allorecognition is basically the capability of an organism that helps in distinguishing
its tissues from those of another organism. This distinguishing is possible within the same
species also plays the important role in the implication of transplantation. Various risks are
there in organ transplantation and one of them is alloresponse, and in this condition,
histoincompatible antigen is identified as well as recognized and it also produces an adaptive
immune response by employing allospecific T-cells (Hauser et al. 2017). All these things can
lead to the direct rejection of all the tissues that are transplanted. But, the direct involvement
of MHC into the mechanism of allorecognition helps in this regard. Here, the T-cell identifies
the determinants on the donor. MHC molecules always display a type of antigenic
determinant that is termed as epitope. T cells have the ability to identify the epitopes
presented by particular allelic variant of MHC molecules. But, if the epitopes are presented
by allelic variants of another MHC molecule, then it is not possible for the T-cells to
recognize those (Van der Meijden et al. 2016).
Tolerance
The development of the T-cells always depends on the interaction with the MHC
molecules. Studies revealed that during the development in the thymus, 98% T cells die due