The document summarizes the HLA system and major histocompatibility complex. It discusses that MHC antigens are located on chromosome 6 and have 4 loci (A, B, C, D). MHC antigens are divided into 3 classes - Class I antigens regulate cytotoxic T cells, Class II antigens regulate helper T cells, and Class III antigens are components of the complement system but not associated with HLA expression. The HLA complex plays important roles in organ transplantation by matching donors and recipients, regulating the immune system through Class I and II antigens, and being associated with certain diseases.

1. HLA SYSTEM AND MAJOR
HISTOCOMPATIBILITY
COMPLEX
T. SOUJANYA
16DC1T0021
PHARM. D

2. INTRODUCTION:
MHC (major histocompatibility complex) is also called as HLA (Human
Leucocyte Antigens) system because these antigens were first discovered on
leucocytes. The major importance of human histocompatibility antigens or HLA
system lies in matching donor and recipient for organ transplant.

3. LOCATION:
Out of various genes for histocompatibility, most of the transplantation antigens
or MHC are located on a portion of chromosome 6 of all nucleated cells of the body
and platelets. This is called “Major histocompatibility complex (MHC) or HLA
system”. These genes occupy four regions or loci-A, B, C and D, on the short (p) arm
of chromosome 6 and exhibit marked variation in allelic genes at each locus.

4. CLASSIFICATION OF MHC
ANTIGENS:
Depending upon the characteristics of MHC,
they have been divided into 3 classes:
1. Class I MHC antigens
2. Class II MHC antigens
3. Class III MHC antigens
HLA system and
loci on
chromosome 6

5. CONTD...
1. Class I MHC antigens: They have loci as HLA-A, HLA-B and HLA-C. CD8+
(i.e. T suppressor) lymphocytes carry receptors for class I MHC and these cells
are used to identify class I antigen on them.
2. Class II MHC antigens: They have single locus as HLA-D. These antigens have
further 3 loci: DR, DQ and DP. Class II MHC is identified by B cells and CD4+
(i.e. T helper) cells.
3. Class III MHC antigens: They are some components of the complementary
system (C2 and C4) coded on HLA complex but are not associated with HLA
expression and are not used in antigen identification.

6. ROLE OF HLA COMPLEX:
The HLA complex is significant in a number of ways:
1. Organ transplantation
2. Regulation of immune system
3. Association of diseases with HLA

7. CONTD...
1. Organ transplantation:
Historically, the major importance of HLA system is in matching donor and
recipient for tissue transplantation.
2. Regulation of immune system:
Class I and II histocompatibility antigens play a role in regulating both cellular
and humoral immunity:
 Class I MHC antigens regulate the function of cytotoxic T cells (CD8+
subpopulation) e.g. In virus infections.
 Class II MHC antigens regulate the function of helper T cells (CD4+
subpopulation).

8. CONTD...
3. Association of diseases with HLA:
An increasing number of diseases have been found to have association with
some specific histocompatibility antigens. These disorders include the following:
i) Inflammatory disorders e.g. Ankylosing spondylitis.
ii) Autoimmune disorders e.g. Rheumatoid arthritis, insulin-dependent diabetes
mellitus (IDDM).
iii) Inherited disorders of metabolism e.g. Idiopathic haemochromatosis.

9. REFERENCE:
A textbook of pathology by Harsh
Mohan