Antibodies may be defined as the proteins that recognize and neutralize any microbial toxin or foreign substance such as bacteria and viruses. The only cells that make antibodies are B lymphocytes. Mainly two forms of antibodies exist. One those that are membrane-bound and act as receptor for antigens on the surface of B lymphocytes and the other that are involved in inhibition of entry and spread of pathogens and are found in blood circulation and connective tissues. The substance or molecule identified by antibodies or that can evoke antibody response is called an antigen .

Some commonly used terminologies

Serum – Clot formation in the blood leaves the residual fluid that contains antibodies. These antibodies in the residue form the serum.

Antiserum – Serum contains a bunch of antibodies and when these antibodies show specificity to a particular antigen by binding to it, those antibodies are known as antiserum.

Serology - Serology may be defined as the study of blood serum or antibodies and their reactions with particular antigens.

7.1 Antibody structure

Antibodies are also called as immunoglobulins and are Y- shaped protein structures. Antibodies consist of two identical light and heavy chains. Amino terminal variable (V) regions are found in both heavy and light chains and they take part in antigen recognition. Effector functions are directed by carboxy – terminal constant (C) regions of the heavy chains but C regions are also found in both the chains. Both the heavy and light chains are composed of Immunoglobulin (Ig) domain. Ig domain is a protein domain that consists of folded repeating units of 110 amino acids in length sandwiched between two layers of β-pleated sheet. The two layers of β–pleated sheet are held together by a disulfide bridge and there are short loops that connect the adjoining strands of each β sheet. Amino acids in some of these loops are most crucial for antigen recognition. Light and heavy chain structure is almost similar. In light chain there is one V region Ig domain and one C region Ig domain whereas in heavy chain V region comprises of one Ig domain and the C region comprises of three or four Ig domains. Antigen-binding site is formed by the V region of one heavy chain and the adjacent V region of one light chain. Disulfide bonds formed between cysteine residues connect the light and heavy chains in the carboxyl terminus of the light chain and the CH-1 domain of the heavy chain. Association of heavy and light chains occurs partly due to the non-covalent interactions between the VL and VH domains and between the CL and CH1 domains. Two heavy chains of each antibody entity are connected covalently by disulfide bonds. In IgG antibodies disulfide bonds are formed between cysteine residues in the CH2 regions which are near to a region known as hinge . This hinge region is more likely to undergo proteolytic cleavage. Fragment antigen binding (Fab fragment) is a portion on antibody that has the capability to bind to antigen and consists of one variable and one constant domain of each of the heavy and the light chain. Fragment crystallizable region (Fc region) is the distal region of an antibody that is composed of two identical, disulfide linked peptides containing the heavy chain CH2 and CH3 domains. Fc region communicates with some cell surface receptors called Fc receptors and this feature of Fc region helps antibodies to stimulate the immune system.