Immunologic methods of diagnostics of hereditary diseases

Recently, the main histocompatibility complex - HLA (Human Leukocyte Antigens) has come to be considered as an important immunological marker of population genetics. The antigens of this system are determined immunologically in blood leukocytes. The HLA gene complex is compactly located on the short arm of chromosome 6 (6p21.3). The localization of this system and the extent of its loci on the chromosome allowed us to calculate that the complex constitutes approximately 1/1000 of the gene pool of the organism. Histocompatibility antigens participate in the regulation of the immune response of the organism, in maintaining immune homeostasis. Due to their polymorphism and compact localization of HLA antigens, they have acquired great importance as a genetic marker.

Currently, more than 200 alleles of this system have been discovered; it is the most polymorphic and biologically significant of the human body's genetic systems. Disorders of various functions of the major histocompatibility complex contribute to the development of a number of diseases, primarily autoimmune, oncological, and infectious.

According to the location of the HLA complex on chromosome 6, the following loci are distinguished: D/DR, B, C, A. New loci G, E, H, F have been discovered relatively recently; their biological role is currently being actively studied. Three classes of antigens are distinguished in the major histocompatibility complex. Class I antigens are encoded by loci A, B, C. New loci also belong to this class. Class II antigens are encoded by loci DR, DP, DQ, DN, DO. Genes of classes I and II encode transplantation antigens. Genes of class III encode complement components (C2, C4a, C4b, Bf), as well as the synthesis of isoforms of a number of enzymes (phosphoglucomutase, glycoxylase, pepsinogen-5, 21-hydroxylase).

The presence of Ag associated with a certain disease in a person allows us to assume an increased predisposition to this pathology, and in some correlations, on the contrary, resistance to it.

Determination of HLA system antigens is carried out on lymphocytes isolated from peripheral blood using histotyping sera in a microlymphocytotoxic reaction or molecular genetic methods.

Establishing associative links between diseases and the antigen of the major histocompatibility complex allows:

• identify groups at increased risk of developing the disease;
• determine its polymorphism, that is, identify groups of patients with features of the course or pathogenesis of the disease; in this regard, an analysis of the syntropy of diseases can be carried out, identifying the genetic prerequisites for the combination of various forms of pathology; association with antigens that determine resistance to diseases allows identifying individuals with a reduced risk of developing this pathology;
• conduct differential diagnostics of diseases;
• determine the prognosis;
• develop optimal treatment tactics.

Since most diseases do not have a direct connection with the antigens of the major histocompatibility complex, the "two-gene" theory was proposed to explain the association between diseases and HLA antigens. According to this theory, there is an immune response gene (genes) (Ir gene) that is closely associated with HLA antigens and genes that regulate the immune response. Protector genes determine resistance to diseases, and provocative genes determine sensitivity to certain diseases.

The relative risk of the disease for individuals with the corresponding genotype is calculated using the formula: x = [h _p × (1 - h _c )] / [h _c × (1 - h _p )], where h _p is the frequency of the trait in patients, and h _c is the frequency in individuals in the control group.

Relative risk shows the magnitude of the association of the disease with a certain Ag/Ag of the HLA system (gives an idea of how many times higher the risk of developing the disease is in the presence of Ag compared to its absence). The higher this indicator in a patient, the higher the associative connection with the disease.

Association of human diseases with HLA-Ag (gene frequency,%)

Diseases	HLA	Control group,%	Sick,%	Relative risk
Rheumatology
Ankylosing spondylitis	B27	5-7	90-93	90-150
Reiter's syndrome	B27	6-9	69-76	32-49.6
Arthritis caused by infections:
- Yersinia	B27		58-76	17.59
- Salmonella	B27		60-69	17.57
Psoriatic arthritis	B13		9-37	4.79
Rheumatoid arthritis	Dw4	12-19	48-72	3.9-12.0
	DR4	20-32	70	4.9-9.33
Behcet's syndrome	B5	13	48-86	7.4-16.4
SKV	B5		11-34	1.83
	B8		19-48	2.11
	Bw15	6-10	21-40	5.1
	DR2	26.4	57.1	3.80
	DR3	22.2	46.4	2.90
Gougerot-Sjogren syndrome	B8		38-58	3.15
	Dw3	26	69-87	19.0
Cardiology
IHD	B7	27.8	45.8	2.19
	B14	7.5	14.8	2.14
	B15	11.1	20.4	2.05
	Cw4	18.7	32.8	2.12
Hypertension	B18	10.4	22.6	2.52
	Aw19	12.6	28.3	2.74
Endocrinology
Type 1 diabetes	B8	32	52-55	2.1-2.5
	B18		5-59	1.65
	B15	12	18-36	1.89-3.9
	Dw3	26	48-50	2.9-3.8
	Dw4	19	42-49	3.5-3.9
	DR3 DR3/DR4	20	60	6.10 33
Hyperthyroidism	B8	21	35-49	2.34-3.5
	D3	26	61	4.4
	DR3	20	51	4.16
Subacute thyroiditis (de Quervain's)	Bw35	13	63-73	16.81
	Dw1		33	2.1
Addison's disease	B8		20-80	3.88-6.4
	Dw3	26	70-76	8.8-10.5
Itsenko-Cushing syndrome	A1		49	2.45
Gastroenterology
Pernicious anemia	B7	19	26-52	1.7-3.1
	DR5	6	25	5.20
Atrophic gastritis	B7		37	2.55
Peptic ulcer of the duodenum	A2	48.1	61.3	1.7
	A10	20.6	63.3	6.65
	B14	4.0	10.3	2.76
	B15	6.6	24.4	4.56
	B40	9.72	23.3	2.82
Autoimmune hepatitis	B8	16	37-68	2.8-4.1
	DR4	24	71	7.75
HBsAg carriers	Bw41		12	11.16
	B15		10-19	0.29

Diseases	HLA	Control group,%	Sick,%	Relative risk
Dermatology
Psoriasis	Bw17	6-8	22-36	3.8-6.4
	B13	3-5	15-27	4.2-5.3
	Bw16	5	15	2.9
Dermatitis herpetiformis	B8	27-29	62-63	4.00-4.6
	DR3	19	80	16.60
Scleroderma	B7	24	35	1.7
Pemphigus	A10			3.1
Atopic dermatitis	B13	6.86	21.28	3.67
	B27	9.94	25.53	3.11
	A10/B13	0.88	8.51	10.48
Eczema	A10	19.64	36.67	2.37
	B27	9.94	26.67	3.29
Urticaria and Quincke's edema	B13	6.86	21,21	3.65
	B5.8	1.42	12,12	9.57
	B5.35	0.71	6.06	9.02
Neurology
Multiple sclerosis	A3	25	36-37	2.7-2.8
	B7	25-33	36-42	1.4-2.0
	Dw2	16-26	60-70	4.3-12.2
	DR2	35	51.2	1.95
	DR3	20	32.5	1.93
Myasthenia	B8	21-24	52-57	3.4-5.0
	A1	20-25	23-56	3.8
	DR3	26	50	2.5
Pulmonology
Bronchial asthma (in patients aged 19-30 years)	B21	4.62	12.5	2.95
	B22	9.94	19.64	2.22
	B27	12.31	37.5	4.27
	B35	0.11	5.36	51.4
	B27/35	0.47	7.14	16.2
Other diseases
Vasomotor rhinitis	A3	26.98	52.38	2.98
	B17	7.57	28.57	4.88
	A3/10	2.72	23.83	11.18
	B7/17	0.47	9.52	22.28

The data presented in the table show that the strongest associative links are found for diseases with a polygenic or multifactorial type of inheritance.

Thus, the determination of antigens of the major histocompatibility complex on blood cells (leukocytes) allows us to identify the degree of individual predisposition of a person to a certain disease, and in some cases use the research results for differential diagnostics, prognosis assessment and selection of treatment tactics. For example, the detection of HLA-B27 antigens is used in the differential diagnostics of autoimmune diseases. It is detected in 90-93% of patients of the Caucasian race with ankylosing spondylitis and Reiter's syndrome. In healthy representatives of this race, HLA-B27 antigens are detected in only 5-7% of cases. HLA-B27 antigens are often detected in psoriatic arthritis, chronic inflammatory bowel diseases occurring with sacroiliitis and spondylitis, uveitis and reactive arthritis.

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