IMMUNOMODULATORS AND THEIR RECEPTORS 3896.1.1.1 Agranular Leukocytes
Lymphocytesare the most common agranular leukocyte. They are small cells, about
8–10μm in diameter, which are almost entirely filled with nucleus. The bloodstream
circulates lymphocytes to every tissue in the body. The task of the lymphocyte is to
specifically recognize unwanted molecular targets and then to set in motion the com-
plex machinery of the immune system. This unwanted molecular target, usually a pro-
tein, is called an antigen, this term being applied to any molecule that is viewed as being
foreign or unwanted by the host. There are two main types of lymphocytes: B lympho-
cytes are the principal mediators for the process of humoral immunity; T lymphocytes
are the principal mediators for the process of cell-mediated immunity.
B Lymphocytes.B lymphocytes are responsible for humoral immunity. Humoral
immunity refers to the production of antibodies(also called immunoglobulins), which
are glycoprotein molecules capable of binding to the antigen. There are five classes
of immunoglobulins, called IgG, IgM, IgA, IgE, and IgD. The basic unit of every
immunoglobulin macromolecule is composed of four peptide chains: two light chains,
and two heavy chains. This basic structure is bifunctional, possessing one fragment (the
F(ab) 2 fragment) that binds to the antigen, and a second fragment (the Fc fragment) that
activates other leukocytes, such as neutrophils, to “zero in” on the targeted F(ab)/antigen
complex, thereby attacking and, hopefully, killing the unwanted cell. Figure 6.2 shows
the structure of an antibody protein.
T Lymphocytes. T lymphocytes are the principal mediators of cell-mediated immunity.
Cell-mediated immunity refers to an immune process that does not directly involve anti-
bodies. In cell-mediated immunity the lymphocyte binds to the unwanted cell and is directly
able to destroy it. Rejection of a kidney transplant is an example of cell-mediated immunity.
Figure 6.2 Immunoglobulins or antibodies are produced by B lymphocytes and facilitate the
destruction of foreign invaders via the humoral component of the adaptive immune system. They
consist of four peptide chains: two heavy, two light. The variable regions of the immunoglobulin
molecule permit binding to the foreign invader.