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  • Antibody Basics

    Antibody: An immunoglobulin, a specialized immune protein, produced because of the introduction of an antigen into the body, and which possesses the remarkable ability to combine with the very antigen that triggered its production.
    The production of antibodies is a major function of the immune system and is carried out by a type of white blood cell called a B cell (B lymphocyte). Antibodies can be triggered by and directed at foreign proteins, microorganisms, or toxins. Some antibodies are autoantibodies and home in against our own tissues.

    Details on Antibody Basics

    The term "antibody" dates to 1901. Prior to that time, an "antibody" referred to any of a host of different substances that served as "bodies" (foot soldiers) in the fight against infection and its ill effects.

    Structure of antibody

    Each antibody consists of four polypeptides- two identical heavy chains and two identical light chains joined to form a "Y" shaped molecule. The amino acid sequence in the tips of the "Y" varies greatly among different antibodies. This variable region, composed of 110-130 amino acids, give the antibody its specificity for binding antigen. The variable region includes the ends of the light and heavy chains. Treating the antibody with a protease can cleave this region, producing Fab or fragment antigen binding that include the variable ends of an antibody.
    Structure of antibody

    Different kinds of antibodies

    There are five diiferent isotypes of antibody depending on their difference in heavy chain. These includes:

    IgG: Heavy chain γ

    This is the principle antibody found in blood and body fluids. Around 75% of the antibody circulating in the blood is IgG, and it is this isotype which provides the majority of the antibody-mediated protection against infection.

    IgG is rarely produced during an initial response to a given pathogen: this antibody is not produced until around one month initial B cell activation. The principle function of IgG is to bind to pathogens via an antigen-specific receptor, and form antigen-antibody complexes. The formation of these complexes targets the pathogen for destruction via other immune cells.

    IgM: Heavy chian μ

    The IgM isotype is expressed on the surface of B cells, but it is also secreted by cells. IgM is typically involved early in an immune response, before B cells have had time to begin secreting large quantities of IgG. This isotype can also trigger the complement cascade, via the same pathway as IgG. However, IgM is a more efficient complement trigger: a single IgM-antigen complex can trigger the cascade, whereas multiple IgG-antigen complexes are required.

    IgA: Heavy chain α

    gA is involved in mucosal immunity, preventing colonization by bacteria in areas which are otherwise unprotected from pathogenic attack. This antibody is found in mucosal areas
    In addition IgA is secreted in tears, saliva, and breast milk. In breast milk, IgA provides nursing infants with passive immunity against pathogens the mother has encountered (as opposed to the active immunity the infant will acquire when its own immune system is activated).

    IgE: Heavy chain ε

    This isotype plays an important role in immune responses to certain parasites, in particular parasitic worms.
    IgE is also strongly associated with allergic reactions, due to its ability to trigger granulocytes to release their toxic chemicals when the antibody comes into contact with its specific antigen. These antigens are proteins from the animal and plant species which commonly trigger allergies.

    IgD: Heavy chain δ

    The function of IgD is not particularly well-defined. This antibody appears as an antigen receptor on naive B cells (those which have not yet been activated by antigen), and provides a signal to B cells at the end of their maturation process in the spleen. Apart from this, IgD has no other known role.

    Fucntions of antibodies

    A. Antigen binding

    Immunoglobulins bind specifically to one or a few closely related antigens. Each immunoglobulin actually binds to a specific antigenic determinant. Antigen binding by antibodies is the primary function of antibodies and can result in protection of the host. The valency of antibody refers to the number of antigenic determinants that an individual antibody molecule can bind. The valency of all antibodies is at least two and in some instances more.

    B. Effector Functions

    Frequently the binding of an antibody to an antigen has no direct biological effect. Rather, the significant biological effects are a consequence of secondary "effector functions" of antibodies. The immunoglobulins mediate a variety of these effector functions. Usually the ability to carry out a particular effector function requires that the antibody bind to its antigen. Not every immunoglobulin will mediate all effector functions. Such effector functions include:
  • 1. Fixation of complement - This results in lysis of cells and release of biologically active molecules
  • 2. Binding to various cell types - Phagocytic cells, lymphocytes, platelets, mast cells, and basophils have receptors that bind immunoglobulins. This binding can activate the cells to perform some function. Some immunoglobulins also bind to receptors on placental trophoblasts, which results in transfer of the immunoglobulin across the placenta. As a result, the transferred maternal antibodies provide immunity to the fetus and newborn