If some foreign substance is injected into the blood vessel of an animal, the injected substance stimulate a specific receptor by special cell in the body, the cell called lymphoid lead to the formation of protein in the animal body called immunoglobulin. These substances are produced as a result stimulation of the body system by the foreign body called antibody. The substance that stimulates the production of antibody is called antigen. Bacterial cell composed of different type of antigen by that the specific immunoglobulin produce in respond to the antigen is called antibody, the substance antibody are mostly found in the serum, a serum that contain antibody is called antiserum. The antibody combines with the corresponding antigen in the body system which will yield antibody antigen reaction, and when this reaction took place, it introduces a valuable resistance to the micro organism causing infection to the host.
Movement of Bacteria: bacteria move with the aid of flagella, there can be a demobilization of the bacteria when antibodies specifically combine with the flagella antigens. Toxic substance produce by these bacteria are detoxify by the antibodies, for example diphtheria toxin, cell bound virulence factor and streptococcal substance can be neutralized when the combine with antibodies. There is another substance in the body system called complement (C) they are specifically found in the protein. Complement help in binding a non specific protein antibody antigen complex and can introduce lyses most of these process usually take place in an antigen of bacteria or other cells. The substance also elevate phagocytosis of bacteria upon the antibody when the antigen in the body absorbed by antigen in the system. The same reactions are fully used by the microbiologist in the everyday diagnosis.
Importantly, microbiologist can used unknown antiserum to detect exact antigen in an unknown micro organism, these happen vice visa e.g. the phenomenon applicable in the diagnosis of typhoid fever, bacteria isolated from stool specimen can be used for examine salmonella typhi. These take place by agglutination reaction using different source of known antisera, some of which where make sure to have antibodies to defend a single factor sera in opposite. For example examination of typhoid fever can be achieved by agglutination reaction where by patient serum are used against known settling substance of salmonella typhi and it counterpart bacteria, paratyphoid fever, the above mechanism used for investigating the typhi and paratyphi of salmonella bacteria is called widal reaction. The confrontation is in circulation, is not satisfactory since each of these bacterial suspension is importantly a mixture of antigen, several of these are share by bacteria that are related, confusion may take place by previous immunization.