Mammalian antibodies are all based on a Y-shaped molecule consisting of four poly peptide chains held together by disulfide bonds (see Figure 1). There are two pairs of chains, known as heavy (H) and light (L). The base of the Y is known as the constant region (C_H1, C_H2 and C_H3) and the tips of the arms are the variable region (V_L, V_H). The amino-acid sequence in the constant region is conserved within a species. The variable region is composed of between 110 and 130 amino acids and varies greatly among different antibodies produced by an individual. Variations in these regions of both the heavy and the light chains form the antigen binding site of the antibody and defi ne its specificity for a particular epitope on the corresponding antigen. Enzymatic digestion of antibodies is performed to generate useful biochemical reagents. Treatment with the enzyme papain gives rise to three fragments: two antigen-binding fragments (Fab) and one constant fragment (Fc). The protease digests the molecule at the hinge region, and the resulting Fab fragments retain their antigen- binding capability. The small-sized Fab fragments can be useful for some immunochemical applications where the larger native antibody molecule would have difficulty binding.

Fig1. Immunoglobulin G.

There are five major classes of antibody molecules, also known as immunoglobulins (Igs). Immunoglobulin G (IgG) is the most common and it is characterised by its Y-shaped structure. The other classes of antibody are immunoglobulins M, A, D and E (Figure 2).

Fig2. Immunoglobulin classes.

Immunoglobulin M ( IgM) is produced by immature and newly activated B lymphocytes in response to initial exposure to an antigen (primary immune response). Structurally it is formed from a circular arrangement of five or six immunoglobulin G molecules. Being a polymer with many binding sites, IgM possesses high avidity. IgM is generally only found in serum as its large size prevents it from crossing tis sue boundaries. The pentameric form is particularly useful for complexing antigens such as bacteria into aggregates, either for disposal or for further processing by the immune system through complement activation. Cells secreting IgM can progress to IgG production, if the animal is challenged again by the antigen. This progression to IgG production is known as affinity maturation and requires maturation of the cells to memory cell status. After several encounters with an antigen, a background level of specific antibody will be found in the blood along with a population of B memory cells capable of rapidly responding to the antigen by initiating high levels of antibody secretion – and thus reaching the status of being immune. Extreme levels of antibody ( hyperimmunity ) may be reached after repeated exposure to an antigen. In this status, additional exposure to antigen can lead to anaphylactic shock due to the overwhelmingly large immune response. Conversely, repeated immunisation can lead to a total loss of immune response to an antigen. This acquired immune tolerance as a response to antigen over-stimulation is characterised by a loss of circulating B cells reactive to the antigen and also by a loss of T cell response to the antigen. This can be used therapeutically to protect individuals against allergic responses.

Immunoglobulin A (IgA) is a dimeric form of immunoglobulin, essentially two IgG molecules arranged end to end with the binding sites facing outwards. This structural topology makes it resistant to enzyme degradation. It is produced by B cells of the mucosal surface of the mouth, nose, eyes, digestive tract and genitourinary system, and is directly secreted into the fluids associated with these tissues, playing a critical role in mucosal immunity.

Immunoglobulin D (IgD) is an antibody resembling IgG and is found on the sur face of immature B cells along with IgM. It is a cellular marker that indicates that an immature B cell is ready to mount an immune response and may be responsible for the migration of the cells from the spleen into the blood. It is used by macrophages to identify the cells to which they can present antigen fragments.

Immunoglobulin E ( IgE) also resembles IgG and is produced in response to allergens and parasites. It is secreted by B lymphocytes and attaches itself to the surface of mast cells and basophils. Binding of allergen to IgE molecules on the cell surface cause the antibodies to cross-link and move together in the cell membrane. This cross-linking triggers the release of histamine . Histamine is responsible for the symptoms suffered by individuals as a result of exposure to allergens.

Immunoglobulin G and, to a lesser extent, IgM are the only two antibodies that are of practical use in immunochemistry. IgG is the antibody of choice for assay development as it is easily purified from serum and tissue culture medium. It is very robust and can be modified by labelling with marker molecules without losing function. It can be stored for extended periods of time at 4 °C or lower. Occasionally, antigens will not generate IgG responses in vivo and IgM is produced instead. This is caused by the antigen being unable to fully activate the B cells and, as a result, no memory cells are produced. Such antigens are often highly glycosylated and it is the large number of sugar residues that block the full activation of the B cells. IgM can be used for assay development, but is more difficult to work with since it is rather unstable. The molecules are difficult to label without losing their function as the presence of labels in adjacent binding sites can easily interfere with antigen recognition due to steric hindrance. IgM can be used directly from cell tissue culture supernatant in assays with an appropriate secondary anti-IgM enzyme conjugate.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم بين الحرمين يقدم جهودًا مضاعفة لإعادة المفقودات إلى الزائرين

بعد برامج تدريبية متخصصة.. متحف الكفيل يبيّن منجزات ملاكاته في فن الخاتم (الأرابيسك)

شعبة مدارس الكفيل الدينية تعقد الاجتماع التحضيري الأول لمهرجان روح النبوّة الثقافي

قسم الشؤون الدينية يقيم دورته الفقهية التطويرية التاسعة لمنتسبي العتبة العباسية المقدسة

المزيد

الآخبار الصحية

ماذا يحدث إذا تناولنا الطعام مرة واحدة فقط في اليوم

اكتشاف عوامل خطر غير متوقعة لمرض الكبد الدهني

فيروس هانتا والسفينة الموبوءة.. تحديث من الصحة العالمية

طبيب يحسم الجدل السجائر الإلكترونية وعلاقتها بالسرطان

المزيد

الآخبار التكنلوجية

لأول مرة في الولايات المتحدة.. تشغيل مفاعل مصغر نووي يغذي الذكاء الاصطناعي بالطاقة

الكشف عن تفاصيل أول عملية ناجحة لاستعادة انتاج الحيوانات المنوية

أطلس يكشف خفايا الجسم البشري بدقة أرفع بـ50 مرة من شعر الإنسان

المتحف المصري الكبير يتحول إلى "أيقونة خضراء" بدعم ياباني

المزيد

مواضيع ذات صلة

Apoptosis in the Immune System

The Major Proinflammatory Cytokines of Innate Immunity: Interleukin-6

The Major Proinflammatory Cytokines of Innate Immunity: Interleukin-1

The Major Proinflammatory Cytokines of Innate Immunity: Tumor Necrosis Factor

The Immune Receptor Family

Collectins and Ficolins

Soluble Effector Molecules of Innate Immunity: Pentraxins

Soluble Effector Molecules of Innate Immunity : The Complement System

MHC Genes

Cytokine-Producing Innate Lymphoid Cells

Epithelial Barriers

C-Type Lectins: Receptors for Microbial Carbohydrates