Each HLA allele has a unique four-, six-, or eight-letter or digit name (Table 31-1). The length of the allele designation depends on the sequence of the allele and that of its nearest relative. All alleles receive a four-letter or digit name; six- and eight-digit names are only assigned when necessary.

Table 31-1

HLA Naming System ^∗

Nomenclature	Indicates
HLA	Human leukocyte antigen (HLA) region and prefix for an HLA gene
HLA-DRB1	Particular HLA locus (e.g., DRB1)
HLA-DRB1^∗13	Group of alleles that encode the DR13 antigen
HLA-DRB1^∗1301	Specific HLA allele
HLA-DRB1^∗1301N	Null allele
HLA-DRB1^∗130102	Allele that differs by a synonymous mutation
HLA-DRB1^∗13010102	Allele that contains a mutation outside the coding region
HLA-A^∗2409N	Null allele
HLA-A^∗3014L	Allele encoding a protein with significantly reduced or low cell surface expression
HLA-A^∗24020102L	Allele encoding a protein with significantly reduced or low cell surface expression, where the mutation is found outside the coding region
HLA-B^∗44020102S	Allele encoding a protein expressed as a secreted molecule only
HLA-A^∗3211Q	Allele that has a mutation previously shown to have a significant effect on cell surface expression, but where this has not been confirmed and its expression remains questionable

^∗As of June 2007, no alleles have been named with the “C” or “A” suffixes.

The first two digits describe the type, which often corresponds to the serologic antigen carried by an allotype. The third and fourth digits are used to list the subtypes, with numbers assigned in the order in which DNA sequences have been determined.

Alleles whose numbers differ in the first four digits must differ in one or more nucleotide substitutions that change the amino acid sequence of the encoded protein. Alleles that differ only by synonymous nucleotide substitutions (also called silent or noncoding substitutions) within the coding sequence are distinguished by the use of fifth and sixth digits. Alleles that only differ by sequence polymorphisms in the introns or in the 5′ or 3′ untranslated regions that flank the exons and introns are distinguished by the use of seventh and eighth digits.

In addition to the unique allele designation, optional suffixes may be added to an allele to indicate its expression status. Alleles shown not to be expressed, termed null alleles, have been given the suffix N. Alleles shown to be alternatively expressed may have the suffix L, S, C, A, or Q.

The suffix L is used to indicate an allele shown to have low cell surface expression compared with normal levels. The S suffix is used to denote an allele specifying a protein that is expressed as a soluble secreted molecule but that is not present on the cell surface. A C suffix indicates an allele product that is present in the cytoplasm but not on the cell surface. An A suffix indicates aberrant expression, where there is some doubt as to whether a protein is expressed. A Q suffix is used when the expression of an allele is questionable, given that the mutation seen in the allele has previously been shown to affect normal expression levels.

Major Histocompatibility Complex Regions

The MHC is divided into four major regions (Table 31-2)—D, B, C, and A. The A, B, and C regions are the classic or class Ia genes that code for class I molecules. The D region codes for class II molecules. Class I includes HLA-A, B, and C. The three principal loci (A, B, and C) and their respective antigens are numbered 1, 2, 3, and so on. The class II gene region antigens are encoded in the HLA-D region and can be subdivided into three families, HLA-DR, HLA-DC (DQ), and HLA-SB (DP).

Table 31-2

Examples of Nomenclature of HLA Alleles

Allele (New Nomenclature)	Frequently Used Shorthand
Class I
HLA-A^∗0101	HLA-A1
HLA-B^∗0801	HLA-B8
Class II
HLA-DRB1^∗0101	HLA-DR1
HLA-DRB1^∗0301	HLA-DR3

Adapted from Peakman M, Vergani D: Basic and clinical immunology, ed 2, New York, 2009, Churchill Livingstone.

Classes of Human Leukocyte Antigen Molecules

Structurally, there are two classes of HLA molecules, class I and class II (Table 31-3). Both classes are cell surface heterodimeric structures. Class I HLA molecules consist of an alpha chain, a highly polymorphic glycoprotein, encoded within the MHC on chromosome 6. This alpha chain noncovalently associates with beta-2 microglobulin, a nonpolymorphic glycoprotein, encoded by a non-HLA gene on chromosome 15. Class II HLA molecules are composed of alpha chains and beta chains encoded within the MHC. The conformation of class I and class II HLA molecules provides each with a groove in which linear peptides, consisting of 8 to 25 peptides, are displayed for recognition by the cell surface expression on lymphocytes of a transmembrane heterodimeric receptor. All nucleated cells of the body display transmembrane class I HLA molecules in association with the non–transmembrane beta-2 microglobulin molecule.

Table 31-3

Comparison of Major Histocompatibility Complex Class I and Class II

Parameter	Class I	Class II
Loci	HLA-A, B, and C	HLA-DN, DO, DP, DQ, and DR
Distribution	Most nucleated cells	B lymphocytes, macrophages, other antigen-presenting cells, activated T lymphocytes
Function	To present endogenous antigen to cytotoxic T lymphocytes	To present endogenous antigen to helper T lymphocytes

MHC, Major histocompatibility complex.

Class I and class II antigens can be found on body cells and in body fluids. Class I and class II molecules are surface membrane proteins. Class I molecules are transmembrane glycoproteins, but the class II dimer molecule differs from class I in that both dimers span the cell membrane. Class I and class II gene products are biochemically distinct, although they appear to be distantly related through evolution. Class III gene products such as C2, C4A, C4B, and Bf complement components are incomplete but these structures are defined by genes lying between or very near the HLA-B and HLA-DR loci.

Multiple alleles occur at each locus. Genes of class I, II, and III antigens at each locus are inherited as codominant alleles. Inheritance within families closely follows simple mendelian dominant characteristics. Conservation of entire haplotypes through generation after generation is the general rule. Very strong linkage disequilibrium is displayed between several HLA loci, creating super or extended haplotypes that may differ from race to race. For example, the most frequent Caucasoid superextended haplotype, AL, Xw7, BB, BfS, C2-1, C4AQOB1, DR3, is almost absent in Asians.

Role of Major Histocompatibility Complex and Human Leukocyte Antigens

The histocompatibility complex that encodes cell surface antigens was first discovered in graft rejection experiments with mice. When the antigens were matched between donor and recipient, the ability of a graft to survive was remarkably improved. A comparable genetic system of alloantigens was subsequently identified in human beings.

The presence of HLA was first recognized when multiply transfused patients experienced transfusion reactions despite proper crossmatching. It was discovered that these reactions were caused by leukocyte antibodies rather than by antibodies directed against erythrocyte antigens. These same antibodies were subsequently discovered in the sera of multiparous women.

The MHC gene products have an important role in clinical immunology. For example, transplants are rejected if performed against MHC barriers; thus, immunosuppressive therapy is required. These antigens are of primary importance and are second only to the ABO antigens in influencing the genetic basis of survival or rejection of transplanted organs.

Although HLA was originally identified by its role in transplant rejection, it is now recognized that the products of HLA genes play a crucial role in our immune system. T cells do not recognize antigens directly but do so when the antigen is presented on the surface of an antigen-presenting cell (APC), the macrophage. In addition to presentation of the antigen, the macrophage must present another molecule for this response to occur. This molecule is a cell surface glycoprotein coded in each species by the MHC. T cells are able to interact with the histocompatibility molecules only if they are genetically identical (MHC restriction).

Both class I and class II antigens function as targets of T lymphocytes that regulate the immune response. Class I molecules regulate interactions between cytolytic T cells and target cells and class II molecules restrict the activity of regulatory T cells (helper, suppressor, and amplifier subsets). Thus, class II molecules regulate the interaction between helper T cells and APCs. Cytotoxic T cells directed against class I antigens are inhibited by CD8 cells; cytotoxic T cells directed against class II antigens are inhibited by CD4 cells. Many genes in both class I and class II gene families have no known functions.

The class I and class II molecules can also bind to self antigens produced in the normal process of cellular protein degradation. Usually, these are not recognized by the T cell receptor (TCR; tolerance). In transplant patients, most immune responses are generated not from bacterial antigens, viral antigens, or self antigens, but from the presentation of alloepitopes derived from the transplanted tissue to circulating T lymphocytes. Two types of alloepitopes are present on transplanted tissue, private and public. Cross-reactive groups have been defined that categorize the cross-reactive alleles of HLA-A and HLA-B.

Class III molecules bear no clear relationship to class I and II molecules aside from their genetic linkage (presence of the gene in or near the MHC complex). Class III molecules are involved in immunologic phenomenon because they represent components of the complement pathways.

Human Leukocyte Antigen Applications

HLA matching is of value in organ transplantation, as well as in the transplantation of bone marrow. The most important HLA antigens are HLA-A, and HLA-B. Everyone has two types of each of these major HLA antigens; there are many different subtypes of HLA-A and of the others. The best possible match is 6/6; the worst possible match is 0/6.

In kidney allografts, the method of organ preservation, the time elapsed between harvesting and transplanting, the number of pretransplantation blood transfusions, the recipient’s age, and the primary cause for kidney failure are all important determinants of early transplantation success or failure. HLA compatibility, however, exerts the strongest influence on long-term kidney survival. The 1-year survival for kidneys transplanted from an HLA-identical sibling approaches 95%. Approximately 50% to 65% of cadaveric kidneys mismatched for all four HLA-A and -B antigens function for 6 months but deteriorate thereafter with time. Only 15% to 25% of these mismatched cadaveric kidneys remain functioning 4 years after transplantation.

It is obligatory to select HLA-identical donors for bone marrow transplantation to reduce the frequency of graft-versus-host disease (GVHD; see later). A method, that depletes donor marrow T cells capable of recognizing foreign host antigens has greatly reduced the incidence of GVHD.

HLA-matched platelets are useful for patients who are refractory to treatment with random donor platelets. In paternity testing, HLA typing is used, along with the determination of ABO, Rh, MNS, Kell, Duffy, and Kidd erythrocyte antigen. In the past, most laboratories involved in testing individuals in disputed parentage cases used only the ABO, Rh, and MNS systems. The chances of identifying a falsely accused man with these tests were 58%. Additional testing for Kell, Duffy, and Kidd erythrocyte antigens and for HLA typing has an exclusion rate estimated at 92%.

HLA typing is also useful in forensic medicine, anthropology, and basic research in immunology. In studies of racial ancestry and migration, some antigens are almost excluded or confined to a race (e.g., A1 and B8 are rarely detected in peoples indigenous to central and eastern Asia, and Bw57 is uncommon in whites and African Americans). These distinctions allow for precise conclusions to be drawn regarding origin and ancestry.

HLA testing has increasingly been used as a diagnostic and genetic counseling tool. Knowledge of HLA antigens and their linkage has become important because of the recognized association of certain antigens (Box 31-1) with distinct immunologic-mediated reactions, autoimmune diseases, some neoplasms, and other disorders; these disorders, although nonimmunologic, are influenced by non-HLA genes also located within the major MHC region.

Box 31-1 Relationship of Certain Human Leukocyte Antigens and Diseases

Ankylosing spondylitis	B27
Reiter’s syndrome	B27
Psoriasis vulgaris	Cw6
Rheumatoid arthritis	DR4
Behçet’s disease	B5 (Bw51)
Type 1 diabetes	DR3
Gold-induced nephropathy	DR5
Congenital adrenal hyperplasia	B47
Chronic lymphatic leukemia	DR5
Kaposi’s sarcoma (Mediterranean)	DR5

The estimated relative risks or chances of developing a disease if a given antigen is present may be elevated in individuals bearing certain HLA antigens compared to individuals who lack the antigen (Table 31-4). The HLA-B27 antigen is the only HLA antigen with a disease association strong enough to be useful in differential diagnosis. Although the degree of association between HLA antigens and other diseases may be statistically significant, it is not strong enough to be of diagnostic or prognostic value.

Table 31-4

Relationship of Human Leukocyte Antigens to Risk of Disease

Antigen Present	Related Disease	Risk^∗
B27	Ankylosing spondylitis Reiter’s syndrome Anterior uveitis Arthritic infection with Yersinia or Salmonella Psoriatic arthritis with spinal involvement Spondylitis associated with inflammatory bowel disease Juvenile chronic arthritis with spinal involvement	100×^† 40× 25× 20× 11× 9× 5×
B8	Celiac disease Addison’s disease Myasthenia gravis Dermatitis herpetiformis Chronic active hepatitis Sjögren’s syndrome Diabetes mellitus (insulin dependent) Thyrotoxicosis	9× 6× 5× 4× 4× 3× 2× 2×
B5	Behçet’s syndrome	6×
BW38	Psoriatic arthritis	7×
BW15	Diabetes mellitus (insulin-dependent)	3×
DR2	Goodpasture’s syndrome Multiple sclerosis	16× 4×
DR3	Gluten-sensitive enteropathy Dermatitis herpetiformis Subacute cutaneous lupus erythematosus Addison’s disease Sjögren’s syndrome (primary)	21× 14× 12× 11× 10×
DR4	Pemphigus^‡ Giant cell arthritis Rheumatoid arthritis Juvenile (insulin-dependent) diabetes mellitus	32× 8× 6× 5×
DR5	Pauciarticular juvenile arthritis Scleroderma Hashimoto’s thyroiditis	5× 5× 3×

^∗Increased risk of developing the disease over a lifetime.

^†Varies with ethnic group (e.g., 3× for Pima Indians and 300× for Japanese).

^‡Jewish persons.

Adapted from Ashman RF: Rheumatic diseases. In Lawlor GJ, Fischer TJ, editors: Manual of allergy and immunology, ed 2, Boston, 1998, Little, Brown.

Although only 8% of normal whites carry the HLA-B27 antigen, 90% of patients with ankylosing spondylitis (AS) or spondylitis in association with Reiter’s syndrome are positive for the antigen. An elevated percentage of HLA-B27–positive patients is also observed in juvenile chronic arthritis with spinal involvement. Therefore, the major indication for screening for HLA-B27 test is to rule out AS when back pain develops in relatives of patients with the disease and to help distinguish incomplete Reiter’s syndrome from gonococcal arthritis, or chronic or atypical Reiter’s syndrome from rheumatoid arthritis. A negative test for HLA-B27, however, does not exclude the diagnosis of AS or Reiter’s syndrome.

Laboratory Evaluation of Potential Transplant Recipients and Donors

Systems developed to ascertain compatibility between donor and recipient include HLA typing and screening of the potential recipient’s serum for the presence of antibodies associated with rejection. Graft success is generally correlated with the presence of a compatible crossmatch, although some transplantation teams have proceeded despite a positive (incompatible) crossmatch.

Human Leukocyte Antigen Typing

A potential recipient needs to have HLA typing (Fig. 31-1, A). A family search may be conducted for a suitable donor. If a suitable match is not found, the patient is placed on a waiting list (see Fig. 31-1, B). When an organ becomes available, the donor is HLA-typed and a computerized search is made for a suitable recipient (see Fig. 31-1, C).

Figure 31-1 Patients (recipients) requiring a solid organ transplant such as a kidney are human leukocyte antigen (HLA)–typed **(A)** and then placed on a transplant registry waiting list **(B).**

C, When a potential organ becomes available, the donor is HLA-typed. If the donor and recipient demonstrate a suitable match by computer, a blood sample is procured from the donor and crossmatched with the recipient’s blood to determine compatibility. D, If no HLA antibodies are detected against the donor cells, the organ is harvested and transplanted to the recipient. (From Nairn R, Helbert M: Immunology for medical students, ed 2, St Louis, 2007, Mosby.)

A newer method of HLA typing is polymerase chain reaction (PCR) amplification of DNA, followed by probing with sequence-specific oligonucleotide probes (SSOPs) and PCR amplification of alleles at loci using allele-specific primers. A simple computer program has been developed to assign the alleles and genotypes based on the probe hybridization pattern.

The difference between HLA-genotyped and zero mismatches reflects the imperfection of the HLA-typing process. HLA genotype–matched means an HLA-identical sibling when all alleles are truly identical. A zero-mismatched unrelated donor may be mismatched because typing does not distinguish between very closely related alleles. In addition, there may be some effect of non-HLA loci because HLA-identical siblings will share only 50% of their minor histocompatibility loci. The degree of donor-recipient mismatch is somewhat obvious, even in the first year.

Flow Cytometry

Single-cell analysis by flow cytometry is the most sensitive method for crossmatching and antibody identification (see Chapter 13). Tagged T or B lymphocytes are incubated with the patient’s serum to allow the formation of antigen-antibody complexes on the cell surface. Unbound proteins are washed away and the bound antibodies are detected with a second antibody, anti–human immunoglobulin G (IgG) labeled with a chromophore. An alternative flow cytometry format uses microparticles coated with HLA antigens of known specificity (obtained through recombinant techniques) instead of lymphocytes.

Facts About Solid Organ Transplantation

Organ transplantation is widely viewed as the preferred treatment for end-stage organ failure because of the quality of life that the treatment offers for patients and because of the long-term cost benefits. The increased demand for organ transplantation is fueled by the transplantation success rate. Worldwide, the demand for transplantation procedures is increasing by about 15%/year, but the number of donated organs has remained static.

The waiting time for allergenic organ transplantation varies widely for many reasons. Each patient’s situation is different. Some patients are more ill than others when they are put on the transplant waiting list. Some patients become sick more quickly than others or respond differently to treatments. Patients may have medical conditions that make finding a good match more difficult.

How long a patient waits for a transplant depends on the following factors:

• Blood type (some rarer than others)

• Tissue type

• Height and weight of transplant candidate

• Size of donated organ

• Medical urgency

• Time on the waiting list

• Distance between donor’s hospital and potential donor organ

• Number of donors in local area over time

• Transplantation center’s criteria for accepting organ offers

Depending on the type of organ needed, some factors are more important than others.

In 1984, the U.S. Congress passed the National Organ Transplant Act. The goal of this legislation was to match a low supply of organs with the most critically ill patients, regardless of where they reside. About 79 patients receive an organ transplant every day in the United States. In 2009, more than 28,000 patients received an organ transplant.

On March 22, 2012, the United Network for Organ Sharing patient waiting list contained 113,612 names. The list continues to grow because of the scarcity of organs (Box 31-2). Most of these registrants are waiting for a kidney transplant, followed by those waiting for a liver transplant and heart transplant. Other transplant registrants are waiting for lung, kidney and pancreas, pancreas, pancreatic islet cell, heart and lung, and intestine. Approximately 25% of patients waiting for a liver transplant are children younger than 10 years.

Box 31-2 Factors Contributing to the Scarcity of Organs

Demand for transplantation has increased because more patients are considered eligible.

Age limits for heart and liver transplants eligibility have increased.

Diabetes is no longer an absolute contraindication for transplant eligibility.

The number of donors has shown little growth.

Adapted from Caplan A: Organ procurement and transplantation: ethical and practical issues, 1995 (www.upenn.edu/ldi/issuebrief2_5.html).

The most common reasons for needing a transplant vary by the type of organ. Kidney recipients usually have diabetes, glomerulonephritis, hypertensive nephrosclerosis, or polycystic kidneys. Liver recipient patients typically have noncholestatic cirrhosis, cholestatic liver disease, biliary atresia, acute hepatic necrosis, or hepatitis C infection. Patients with cardiomyopathy, congenital heart disease, valvular heart disease, or coronary artery disease are the most frequent heart transplant recipients.

The number of patients living with a function graft has generally increased over the last decade. Graft survival time depends on many factors, including the type of organ transplanted (Fig. 31-2 and Table 31-5).

Table 31-5

Selected Examples of Single Organ Graft Transplant Survival (Unadjusted Graft Survival Expressed in Percentage (%))

Follow–up Period
	3 Months	1 Year	3 Years	5 Years	10 Years
	TX 2007-2008	TX 2007-2008	2005-2008	2003-2008	1998-2008
Kidney: Deceased Donor Kidney: Living Donor	95.6 98.0	91.7 96.5	81.8 90.5	70.8 82.8	44.9 61.2
Pancreas	85.0	74.8	62.6	52.0	35.1
Liver: Deceased Donor	91.7	85.2	75.1	68.5	54.8
Liver: Living Donor	91.3	88.2	80.1	74.6	59.6
Intestine	91.3	81.7	55.2	41.5	24.0
Heart	93.1	88.3	80.7	73.9	54.7
Lung	91.6	81.5	63.3	51.2	26.1

Tx = year or inclusive years of tranplantation.

Source: Organ Procurement and Transplantation Network (OPTN) and Scientific Registry of Transplant Recipients (SRTR). OPTN / SRTR 2010 Annual Data Report. Rockville, MD: Department of Health and Human Services, Health Resources and Services Administration, Healthcare Systems Bureau, Division of Transplantation; 2011. www.srtr.org/annual_reports/2010

Figure 31-2 Number of surviving patients after kidney or liver transplants from deceased or living donors. (From Organ Procurement and Transplantation Network (OPTN) and Scientific Registry of Transplant Recipients (SRTR). OPTN / SRTR 2010 Annual Data Report. Rockville, MD: Department of Health and Human Services, Health Resources and Services Administration, Healthcare Systems Bureau, Division of Transplantation; 2011, www.srtr.org/annual_reports/2010.)

Transplantation Terminology

The transplanting or grafting of an organ or tissue ranges from self-transplantation, such as skin grafts from one part of the body to another to correct burn injuries, or hair transplants from one area of the scalp to another to correct pattern baldness, to the grafting of a body component from one species to another, such as transplanting a pig’s heart valve to a human. Table 31-6 defines the most recent terms used in transplantation.

Table 31-6

Transplantation Terms

Term	Definition
Autograft	Graft transferred from one position to another in the same individual (e.g., skin, hair, bone)
Syngraft	Graft transplanted between different but identical recipient and donor (e.g., kidney transplant between monozygous twins)
Allograft (homograft)	Graft between genetically different recipient and donor of the same species; grafted donor tissue or organ contains antigens not present in recipient
Xenograft (heterograft)	Graft between individuals of different species (e.g., pig heart valve to a human heart)

Types of Transplants

Eleven different organs or human body parts can be transplanted—blood vessels, bone, bone marrow or stem cells (see Chapter 32), cornea, heart, kidneys, liver, lung, middle ear, pancreas, and skin. Successful organ transplants have increased since the advent of the immunosuppressive drug cyclosporine (cyclosporin A).

Living donor transplants have attracted significant media attention. According to the United Network for Organ Sharing and the Health Resources and Services Administration of the U.S. Department of Health and Human Services, a living donor may donate a single kidney, segment of the liver, portion of the pancreas, or the lobe of a lung.