Serology is simply defined as the study of in vitro reaction between antigen and antibody. The word “In vitro” means “reaction occurring outside the body of a living organism”, that is in plate e.g. Petri dish plate. This reaction is opposed to in vivo reaction, which is a reaction that occur inside the body of a living organism. Most or majority of the test carried out in the microbiology laboratory are in – vitro test, in which the causative agent of a particular infection is deciphered by obtaining a specimen from a sick patient and testing it in the laboratory so that appropriate treatment plan can be organized or put in place for the patient experiencing the pathologic state.
In vivo test/experiments are usually employed in research laboratories, where animals are used for carrying out research. The serology unit of the clinical microbiology laboratory is solely responsible for testing patient’s serum and plasma in order to detect antibodies produced by the body in response to an infection. Serological techniques are usually used for the surveillance and early detection of diseases caused by microorganisms that are too difficult to culture in vitro.
Antibody is defined as a protein molecule synthesized/produced by the immune system on exposure of the body to an antigen. According to their physiological and biochemical properties, there are 5 different classes of antibodies produced by the human body: Immunoglobulin (Ig) A, D, G, E, and, M.
Antigen is defined as a foreign molecule/substance that can induce/start an immune response in our body. Typical examples of antigens are: various pathogens (bacteria, virus, protozoa, and fungi) that cause diseases in humans.
Every living organism especially man is constantly being attacked by one disease causing germs (pathogens) or the other in its lifetime. Man and other animals have evolved a strategy called “IMMUNE SYSTEM”, which they use to respond to these pathogens and evade their future threat to health. The immune system helps to free the body from microbe – burden. When an organism develops immunity (protection) to a specific pathogen after prior infection with same pathogen, the organism will remain free from that agent/pathogen for life.
This is the beautiful nature of the immune system which GOD blessed us with. Also very important is the segregative nature of our immune system. The immune system has the ability to different antigens (non – self) from the tissues, organs, and cells (self) in our body, so that the antibody it produces will only attack and fight the antigen/foreign substances that entered our body. Thus, antibodies are specific in their activities, binding only onto antigens. When this specificity of the immune system fails, an autoimmune disease/reaction occurs – in which the body starts fighting against itself.
The Rationale behind the use of serological tests in the laboratory diagnosis of an infection is that: when an infection occurs in the body of an organism, antibodies will be produced by the immune system against the antigen/pathogen responsible for the pathologic state of the patient. After the antibodies produced must have gone into action fighting and killing the antigen, they will die and remain in the body of the patient.
Now, to know whether such infection really took place, a preparation of the infecting pathogen which will serve as an antigen will be mixed or tested on a plate/test card with the serum/plasma obtained from the patient’s blood. A positive reaction shows that there was indeed an infection, while a negative reaction rules out the possibility of any infection. This is the basis for serological testing’s in the clinical microbiology laboratory, knowing the fact that both antigens and antibodies react specifically. Some of the serological tests used in the clinical microbiology laboratory will be the focus of the proceeding discussions. Enjoy yourself!