Immunochemistry

The immune system The body has two different types of immune response: 1) if the response to an antigen (foreign body) is the production of soluble antibodies in the body fluids, it is called: Humoral immunity. 2)if the response is through cytotoxic or killer T cells, then the immunity is known as cell-mediated. These two mechanisms complement each other. The challenge for the immune system is to be able to provide antibodies to interact with the antigens or foreign cells (both known as non-self) that invade the body, but not to interact with the body’s own proteins (known as self).

Antibodies (Immunglobulins (Igs) A- Definition:  Immunoglobulins are a group of proteins (gamma globulins) produced by the body from B lymphocytes and plasma cells in response to presence of foreign bodies.  There are 5 basic types: G, M, A, E and D.  All of immunoglobulins have a similar basic structure. B- Basic structure of immunoglobulins:  The basic unit of all Igs molecules consists of 4 polypeptide chains linked by disulfide bonds: a- 2 polypeptide chains of low molecular weight called light chains (L) a- 2 polypeptide chains of low molecular weight called light chains (L) b- 2 polypeptide chains of high molecular weight called heavy chains (H) b- 2 polypeptide chains of high molecular weight called heavy chains (H)

 Both H and L chains have 2 regions a- C-terminal constant regions: having a constant amino acid sequence within a class or type. a- C-terminal constant regions: having a constant amino acid sequence within a class or type. b- N-terminal variable (v) regions: with considerable variation in amino acid sequence from molecule to another. b- N-terminal variable (v) regions: with considerable variation in amino acid sequence from molecule to another.  Antigen- binding sites: This the part of antibody molecule which combines with antigens. It is formed by a few amino acids in the variable (v) region of H and L chains at the N- terminal.  Enzymes such as papain cleave IgG into 3 fragments: 2 Fab fragments and one Fc fragment. The Fab Fragment is the part of the antibody molecule which combines with antigen.

Classes of L and H chains: Classes of L and H chains: a- Only 2 major types of L chains in man, Kappa (k) and lambda (λ) chains. Approx. 70% of the human Ig molecules carry k light chains and 30% carry λ light chains. a- Only 2 major types of L chains in man, Kappa (k) and lambda (λ) chains. Approx. 70% of the human Ig molecules carry k light chains and 30% carry λ light chains. b- H chain is unique to the class. In IgG, the H chains is termed γ chain, in IgA, an α chain. In IgM a μ chain, in IgD, a δ chain and an έ chain. b- H chain is unique to the class. In IgG, the H chains is termed γ chain, in IgA, an α chain. In IgM a μ chain, in IgD, a δ chain and an έ chain.  Igs contain CHO residues (2 to 12%). These are either mannose, galactose, fructose or N-acetyl neuraminic acid. They are attached to polypeptide chains.

 Types of immunoglobulins: 1. IgG: a- It forms 70-75% of the total serum Igs. b- It is the only Ig which can cross the placenta. b- It is the only Ig which can cross the placenta. c- It is the major Ig during secondary immune response (i.e. prolonged stimulation of lymphocytes) with long half life (about 30 days) if compared with other immunoglobulins. c- It is the major Ig during secondary immune response (i.e. prolonged stimulation of lymphocytes) with long half life (about 30 days) if compared with other immunoglobulins. d- It is of low molecular weight formed of one unit only (monomer), and contain 2-4% CHO. d- It is of low molecular weight formed of one unit only (monomer), and contain 2-4% CHO. 2. IgA: a- in serum present as monomer. In body fluids as diamer. In diameric IgA, the molecules are joined by a small polypeptide chain called J chain linked to Fc regions.

3. IgM: a- It is present in serum and has the highest molecular weight among all Igs as it is formed of 5 basic subunits (pentamer) b- Ig subunits are joined together by disulfide bonds in a circular-fashion to form star. b- Ig subunits are joined together by disulfide bonds in a circular-fashion to form star. c- It is the major Ig during the primary immune response i.e. it is thr first of the antibodies which act on introduction of foreign antigen into plasma. Its present indicates recent infection. c- It is the major Ig during the primary immune response i.e. it is thr first of the antibodies which act on introduction of foreign antigen into plasma. Its present indicates recent infection. d- It can not cross the placenta. d- It can not cross the placenta. e- IgM contains 10% CHO and has a realatively short half life. e- IgM contains 10% CHO and has a realatively short half life. 4. Ig E: a- It is present in very low concentration in serum and has the shortest half life (2-3 days). b- It is largely responsible for immunity against parasites

c- Ig E,in the presence of antigen, binds to mast cells and basophils causing release of histamine and other substances from mast cells. These substances result in allergic manifestations. c- Ig E,in the presence of antigen, binds to mast cells and basophils causing release of histamine and other substances from mast cells. These substances result in allergic manifestations. 5. Ig D: a- It is present in a very low concentration in serum. b- It can not across placenta. b- It can not across placenta. c- Ig D has activity against thyroid tissue, insulin, penicillin and diphtheria toxoid c- Ig D has activity against thyroid tissue, insulin, penicillin and diphtheria toxoid

Antigens (Immunogens) Antigens (Immunogens) Definition: Definition: Antigens are substances when introduced to the body will stimulate an immune response i.e. antibodies production. Proteins, polysaccharides and other synthetic polymers are good antigens. Lipids are not antigenic unless they are combined with proteins or polysaccharides. Haptens: are small molecules that can not by themselves induce antibody formation but can do so when covalently linked to larger molecules Haptens: are small molecules that can not by themselves induce antibody formation but can do so when covalently linked to larger molecules Hapten + lymphocytes → No antibodies Hapten + lymphocytes → No antibodies Hapten + protein + lymphocytes → antibodies against hapten Hapten + protein + lymphocytes → antibodies against hapten

Properties of antigens: 1- High molecular weight: thus proteins of molecular wt greater than 100,000 are the most potent antigens. 2- Foreign to the body: Under normal conditions, tissues or fluids of the body can be recognized by immune system as self ( i.e. own tissues), and so do not stimulate immune response. If foreign substances are introduced in the body, immune system recognizes them as nonself and immune response will occur. If foreign substances are introduced in the body, immune system recognizes them as nonself and immune response will occur. 3- Structural complexity: A molecule must posses a certain degree of complexity to be antigenic. Immunogenicity increases with structural complexity. 3- Structural complexity: A molecule must posses a certain degree of complexity to be antigenic. Immunogenicity increases with structural complexity.

Initiation of Ig production requires binding of the antigen to the lymphocyte surface: Initiation of Ig production requires binding of the antigen to the lymphocyte surface: Antigen + lymphocyte surface → Ig  Antigens are bound to antigen receptors located on the surface of lymphocytes.  In other words, presence of antigen stimulates and activates B lymphocytes which are then be mature and differentiated into plasma cells which produce antibodies.  Antigenic determinant: This the portion of antigen that binds with the antigen receptors.  Immunopotency: It is the capacity of the region of the antigenic determinant to induce the formation of specific antibodies.

Neonatal and infant antibody production: Neonatal and infant antibody production: The human fetus is able to synthesize only a small amount of Ig M and to a lesser degree, Ig A. Ig G has lower molecular wt than Ig M and is easily able to cross the placenta.. Ig G is also transferred from mother to baby in breast milk. Transplacental and\breast milk-derived Ig G offer the baby a passive immunologic protection until the endogenous Ig G production takes place. The half life of Ig G is about 30 days and with prolonged breast –feeding, the infant can derive additional protection. The human fetus is able to synthesize only a small amount of Ig M and to a lesser degree, Ig A. Ig G has lower molecular wt than Ig M and is easily able to cross the placenta.. Ig G is also transferred from mother to baby in breast milk. Transplacental and\breast milk-derived Ig G offer the baby a passive immunologic protection until the endogenous Ig G production takes place. The half life of Ig G is about 30 days and with prolonged breast –feeding, the infant can derive additional protection.

Immunity disorders: 1- Overproduction of Igs (hypergammaglonlinemia): Due to increased synthesis. There are two types: a- Diffuse hypergammaglobulinemia: when all Ig classes are increased. a- Diffuse hypergammaglobulinemia: when all Ig classes are increased. b- Discrete hypergammaglobulinemia (paraproteinemia): when a single Ig or Ig fraction ( e.g. light chain) is increased. b- Discrete hypergammaglobulinemia (paraproteinemia): when a single Ig or Ig fraction ( e.g. light chain) is increased. 2- Underproduction of Igs (Hypogammaglobulinemia): It may be congenital or acquired due to delayed onset of Ig production of unknown etiology leading to immunodeficiency and recurrent infection.

3- Agammaglobulinemia (Bruton’s disease): Complete absence of gamma globulins occur in boys (X-linked disorder) in which patients have no B lymphocytes and so low levels (or none) of all endogenous Ig subclasses are produced. The disease leads to recurrent febrile infection in early life, most infection in lower and upper respiratory system resulting in otitis, pneumonia, sepsis, osteomyelitis (inflammation of bone marrow) and meningitis. Without early γ globulin therapy, these children die from respiratory complications. Other immune pathways are normal in these children.

4- Severe Combined Immune Deficiency (SCID): occur in infants who lack both humoral (Abs) and cellular pathways (NK cells) for killing bacteria and viruses. These children are at risk of severe infection each time they are exposed to infectious agents. The disease may be X-linked (in boys only) or autosomal ( inherited in girls also). Causes: i- aplasia of B. M. or Failure of lymphocyte development and maturation in which both T-cells and B cells (if present) are not functional. ii- genetic disorder of purine metabolism: genetic deficiency of adenosine deaminase leading to elevated levels of adenosine in body fluids leading to failure of DNA synthesis and so no protein synthesis. In this case the treatment is enzyme replacement therapy or gene therapy. Main treatment: 1-boy in the bubble i.e. in complete sterile environment to avoid contact with any bacteria or viruses. 2- Bone marrow replacement. 3- interferon may be useful

 Other components of immune system 1- Skin: is effective barrier to most microorganisms, but in premature babies, this barrier is less well developed 2- Lysozyme: is an enzyme widely distributed in different secretions and is capable of partially digesting a chemical bond in the membrane of many bacterial cell wall. 3- Macrophages: In the presence of foreign organism, monocytes are derived from B. M. and then are developed into macrophages that subsequently engulf and digest organism. 4- Natural killer (NK)cells or cell-mediated immunity: which are leucocytes (lymphocytes) capable of recognizing cell surface changes on host cells infected by virus particles. The infected host cells release interferon which stimulates NK cells production which subsequently bind to these host cells and kill virus.

Interferon is also capable of providing resistance to infection in host cells not virally infected. 5- Acute Phase proteins: which are defense proteins produced by the liver in response to infection, particularly bacterial infection. The most significance acute phase protein is called ‘ C-reactive protein’ (CRP). Letter C is added as it is capable of binding to C-protein of pneumococci. Acute phase proteins are the main components of complement. Complement system: consists of at least 20 proteins most of which are acute phase proteins. Complement is produced as a result of antigen-antibody reaction and aids antibodies in killing foreign bodies. Complement is produced as a result of antigen-antibody reaction and aids antibodies in killing foreign bodies. So, antibodies, acute phase proteins and complement coordinate with each other to ultimately destroy bacteria and viruses.