Histocompatibility Antigens

Nucleated cells such as leukocytes and tissues possess many cell surface–protein antigens that readily provoke an immune response if transferred into a genetically different (allogenic) individual of the same species. Some of these antigens, which constitute the major histocompatibility complex (MHC) (see Color Plate 2), are more potent than others in provoking an immune response. The MHC is referred to as the human leukocyte antigen (HLA) system in humans because its gene products were originally identified on white blood cells (WBCs, leukocytes). These antigens are second only to the ABO antigens in influencing the survival or graft rejection of transplanted organs. HLAs are the subject of numerous scientific investigations because of the strong association between individual HLAs and immunologic disorders (see Chapter 31 for more discussion of the MHC).

Major Histocompatibility Complex Regions

The MHC is divided into four major regions (Fig. 2-1)—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, HLA-B, and HLA-C. The three principal loci (A, B, and C) and their respective antigens are numbered, for example, as 1, 2, 3. 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).

Classes of HLA Molecules

Structurally, there are two classes of HLA molecules, class I and class II (Table 2-1). Both class I and class II antigens function as targets of T lymphocytes (see Chapter 4 for a further discussion of lymphocytes) that regulate the immune response (Fig. 2-2). Class I molecules regulate interaction between cytolytic T cells and target cells and class II molecules restrict the activity of regulatory T cells. Thus, class II molecules regulate the interaction between helper T cells and antigen-presenting cells (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 the class I and class II gene families have no known function.

Table 2-1

Comparison of MHC Class I and Class II

	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

Autoantigens

The evolution of a recognition system that can recognize and destroy nonself material must also have safeguards to prevent damage to self antigens. The body’s immune system usually exercises tolerance to self antigens but, in some situations, antibodies may be produced in response to normal self antigens. This failure to recognize self antigens can result in autoantibodies directed at hormones, such as thyroglobulin (see Chapter 28).

Blood Group Antigens

Blood group substances are widely distributed throughout the tissues, blood cells, and body fluids. When foreign RBC antigens are introduced to a host, a transfusion reaction or hemolytic disease of the fetus and newborn can result (see Chapter 26). In addition, certain antigens, especially those of the Rh system, are integral structural components of the erythrocyte (RBC) membrane. If these antigens are missing, the erythrocyte membrane is defective and results in hemolytic anemia. When antigens do not form part of the essential membrane structure (e.g., A, B, and H antigens), the absence of antigen has no effect on membrane integrity.

Adjuvant

The response to immunization can be enhanced by a number of agents, collectively called adjuvants. One of the best-known emulsifying agents in vaccine studies is Freund’s complete adjuvant. An adjuvant is a substance, distinct from antigen, that enhances T cell activation by promoting the accumulation of APCs at a site of antigen exposure and by enhancing the expression of costimulators and cytokines by the APCs.

Foreignness

Foreignness is the degree to which antigenic determinants are recognized as nonself by an individual’s immune system. The immunogenicity of a molecule depends to a great extent on its degree of foreignness. For example, if a transplant recipient receives a donor organ with several major HLA differences, the organ is perceived as foreign and is subsequently rejected by the recipient. Normally, an individual’s immune system does not respond to self antigens.

Degradability

For an antigen to be recognized as foreign by an individual’s immune system, sufficient antigens to stimulate an immune response must be present. Foreign molecules are rapidly destroyed and thus cannot provide adequate antigenic exposure. In the case of vaccination, an adequate dose of vaccine at appropriate intervals must be administered for an immune response to be stimulated.

Molecular Weight

The higher the MW, the better the molecule will function as an antigen. The number of antigenic determinants on a molecule is directly related to its size. For example, proteins are effective antigens because of a large MW.

Although large foreign molecules (MW 10,000 daltons [Da]) are better antigens, haptens, which are tiny molecules, can bind to a larger carrier molecule and behave as antigens. If a hapten is chemically linked to a large molecule, a new surface structure is formed on the large molecule, which may function as an antigenic determinant.

Structural Stability

If a molecule is an effective antigen, structural stability is mandatory. If a structure is unstable (e.g., gelatin), the molecule will be a poor antigen. Similarly, totally inert molecules are poor antigens. Ther structural stability of an antigen is important in cases where the goal is to elicit a patient antibody response when adminstering a vaccine.

Complexity

The more complex an antigen, the greater is its effectiveness. Complex proteins are better antigens than large repeating polymers such as lipids, carbohydrates, and nucleic acids, which are relatively poor antigens.

Immunoglobulin M

Immunoglobulin M accounts for about 10% of the Ig pool and is largely confined to the intravascular pool because of its large size. This antibody is produced early in an immune response and is largely confined to the blood. IgM is effective in agglutination and cytolytic reactions. In humans, IgM is found in smaller concentrations than IgG or IgA. The molecule has five individual heavy chains, with an MW of 65,000 Da; the whole molecule has an MW of 900,000 Da and sedimentation coefficient, Σ, of 19.

Normal values of IgM are 60 to 250 mg/dL (70 to 290 IU/mL) for males and 70 to 280 mg/dL (80 to 320 IU/mL) for females. At 4 months of age, 50% of the adult level is present; adult levels are reached by 8 to 15 years. Cord blood contains greater than 20 mg/dL. IgM is usually undetectable in cerebrospinal fluid (CSF).

IgM is decreased in primary (genetically determined) Ig disorders as well as secondary Ig deficiencies (acquired disorders associated with certain diseases). IgM can be increased in the following conditions:

Immunoglobulin G

The major immunoglobulin in normal serum is IgG. It diffuses more readily than other immunoglobulins into the extravascular spaces and neutralizes toxins or binds to microorganisms in extravascular spaces. IgG can cross the placenta. In addition, when IgG complexes are formed, complement can be activated. IgG accounts for 70% to 75% of the total Ig pool. It is a 7S molecule, with an MW of approximately 150,000 Da. One of the subclasses, IgG3, is slightly larger (170,000 Da) than the other subclasses.

Normal human adult serum values of IgG are 800 to 1800 mg/dL (90 to 210 IU/mL). In infants 3 to 4 months old, the IgG level is approximately 350 to 400 mg/dL (40 to 45 IU/mL), gradually increasing to 700 to 800 mg/dL (80 to 90 IU/mL) by the end of the first year of life (Fig. 2-4). The average adult level is achieved before age 16 years. Other body fluids containing IgG include cord blood (800 to 1800 mg/dL) and CSF (2 to 4 mg/dL).

Decreased levels of IgG can be manifested in primary (genetic) or secondary (acquired) Ig deficiencies. Significant increases of IgG are seen in the following conditions:

Immunoglobulin A

Immunoglobulin A represents 15% to 20% of the total circulatory Ig pool. It is the predominant immunoglobulin in secretions such as tears, saliva, colostrum, milk, and intestinal fluids. IgA is synthesized largely by plasma cells located on body surfaces. If produced by cells in the intestinal wall, IgA may pass directly into the intestinal lumen or diffuse into the blood circulation. As IgA is transported through intestinal epithelial cells or hepatocytes, it binds to a glycoprotein called the secretory component. The secretory piece protects IgA from digestion by gastrointestinal proteolytic enzymes. It forms a complex molecule termed secretory IgA, which is critical in protecting body surfaces against invading microorganisms because of its presence in seromucous secretions (e.g., tears, saliva, nasal fluids, colostrum).

IgA monomer is present in relatively high concentrations in human serum; it has a concentration of 90 to 450 mg/dL (55 to 270 IU/mL) in normal adult humans. At the end of the first year of life, 25% of the adult IgA level is reached, and 50% at 3.5 years of age. The average adult level is attained by age 16 years. IgA concentration in cord blood is greater than 1 mg/dL; CSF contains 0.1 to 0.6 mg/dL of IgA.

IgA is decreased in primary or secondary Ig deficiencies. Significant increases in serum IgA concentration are associated with the following:

Immunoglobulin D

Immunoglobulin D is found in very low concentrations in plasma, accounting for less than 1% of the total Ig pool. IgD is extremely susceptible to proteolysis and is primarily a cell membrane Ig found on the surface of B lymphocytes in association with IgM.

Immunoglobulin E

Immunoglobulin E is a trace plasma protein found in the blood plasma of unparasitized individuals (MW, 188,000 Da). IgE is crucial because it mediates some types of hypersensitivity (allergic) reactions, allergies, and anaphylaxis and is generally responsible for an individual’s immunity to invading parasites. The IgE molecule is unique in that it binds strongly to a receptor on mast cells and basophils and, together with antigen, mediates the release of histamines and heparin from these cells.

Typical Immunoglobulin Molecule

The basic unit of an antibody structure is the homology unit, or domain. A typical molecule has 12 domains, arranged in two heavy (H) and two light (L) chains, linked through cysteine residues by disulfide bonds so that the domains lie in pairs (Fig. 2-5). The antigen-binding portion of the molecule (N-terminal end) shows such heterogeneity that it is known as the variable (V) region; the remainder is composed of relatively constant amino acid sequences, the constant (C) region. Short segments of about 10 amino acid residues within the variable regions of antibodies (or T cell receptor [TCR] proteins) form loop structures called complementary-determining regions (CDRs). Three hypervariable loops, also called CDRs, are present in each antibody H chain and L chain. Most of the variability among different antibodies or TCRs is located within these loops.