The Collaborative International Dictionary of English v.0.48:

immunoglobulin \im`mu*no*glob"ulin\
   ([i^]m`m[-u]*n[-o]*gl[o^]b"[-u]*l[i^]n) n. (1953)
   any one of a class of globular proteins which are antibodies
   and are produced by the immune system in animals.

   Note: The immunoglobulins form a series of related proteins
         which are each composed of two pairs of polypeptide
         chains, called heavy (H) and light (L, meaning of lower
         molecular weight), all linked together by disulfide
         bonds. They are subdivided on the basis of the
         structural and antigenic properties of the H chains
         into four subgroups, immunoglobulin G (IgG),
         immunoglobulin A (IgA), immunoglobulin M (IgM), and
         immunoglobulin D (IgD). They are divided also into
         subclasses. Both H and L chains of anny given class and
         subclass have regions which are of constant structure
         within that class, as well as regions which are of
         variable structure. The variable regions impart the
         ability to recognize and bind to specific molecular
         structures, thus providing the organism the capacity to
         recognize and defend itself against the harmful effects
         of substances foreign to the body. --Stedman

   Syn: Ig.
        [PJC]

WordNet (r) 3.0 (2006):

immunoglobulin A
    n 1: one of the most common of the five major classes of
         immunoglobulins; the chief antibody in the membranes of the
         gastrointestinal and respiratory tracts [syn:
         immunoglobulin A, IgA]