Immunohistochemistry is a technique that uses antibodies to identify antigens in cells and tissues. It works by binding antibodies, which are highly specific serum proteins, to antigens in the tissue. This binding is then identified either directly by labeling the antibody or through a secondary labeling method. It is used to study antigen expression, antigen status of cells, and antigen location in tissues. Antibodies are produced against antigens, which are molecules that induce antibody formation. There are two main types of antibodies used: polyclonal antibodies which are a heterogeneous mixture, and monoclonal antibodies which are a homogeneous population against a single epitope. Labels like enzymes, metals, fluorescent dyes or radioisotopes are used to identify the antigen-antibody complex

1. IMMUNOHISTOCHEMISTRY

2. INTRODUCTION
Immunohistochemistry is a technique for identifying cellular or tissue
constituents (antigens) by means of antigen antibody interactions. The
site of antibody binding is identified either by direct labeling of the
antibody, or by use of a secondary labeling method.
It is used for
i. Expression of a particular antigen in a tissue
ii. Antigenic status of particular cells in a tissue
iii. Microanatomic location of an antigen
It uses antibodies to distinguish antigenic differences between cells

3. ANTIGEN
An antigen is a molecule that induces the formation of
an antibody and bears one or more antibody binding
sites.
These binding sites are highly specific topographical
regions composed of a small number of amino acids or
monosaccharide units, being known as antigenic
determinant groups or epitopes.

4. ANTIBODY
Antibodies belong to the class of serum proteins known as
immunoglobulins. The terms antibody and immunoglobulin are often
used interchangeably. They are found in blood and tissue fluids, as well
as many secretions.
The basic unit of each antibody is a monomer. The monomer is
composed of two heavy and two light chains. An antibody can be
monomeric, dimeric, trimeric, tetrameric, or pentameric. There are five
types of antibody found in the blood of higher vertebrates: IgA, IgD, IgE,
IgG, and IgM.
IgG is the commonest and the most frequently used antibody for

6. Two types of antibodies are used:
 Polyclonal antibodies
 Monoclonal antibodies
Polyclonal antibodies
 They are a heterogenous mixture of antibodies directed against various
epitopes of same antigen.
 Generated by different B-cell clones of the animals that are
immunochemically dissimilar.
Monoclonal antibodies
 They are a homogenous population of lg directed against a single epitope.
 Generated by a single B-cell clone from one animal that are
immunochemically similar.

7. LABEL
A label is a molecule which aids in the identification of the antigen-
antibody complex.
Types of label:
i. Enzyme labels
Enzymes are the most widely used labels in immunohistochemistry,
and incubation with a chromogen using a standard histochemical
method produces a stable, colored reaction end-product suitable
for the light microscope.
Horseradish peroxidase (HRP) is the most widely used enzyme, and in
combination with the most favored chromogen, i.e. 3,3 α-
diaminobenzidine tetrahydrochloride (DAB).
ii. Colloidal metal labels
iii. Fluorescent labels
iv. Radiolabels