The immune system is a remarkably variable defense system that has evolved to protect animals from invading pathogenic microorganisms. Functionally an immune response can be divided into two related activities: recognition and response. The immune system is able to recognize subtle chemical differences that distinguish one pathogen from the another. Once foreign organism recognize the immune system, give response called effecter response to eliminate or neutralize the organisms. The ability to resist disease may be innate (non-specific) or it may be acquired. It is inborn and provides the basic mechanisms that defend the host against intrusion of foreign substance or agent of disease. It also provides the body to resist much pathological condition. The mechanism involved in this immunity are mechanical barrier, such as skin and mucous membranes, biochemical factors (antimicorbial agent present inside the body fluids) etc.
Specific immunity (cell mediated and humoral) acquired by the host in response to presence of single or particular foreign substances (protein) called antigen.
In case of humoral immunity, specific antibodies are produced against antigen which binds to the specific antigen that is responsible of their production. Antibodies are group of homologous proteins called immunoglobilin. Antigen, antibody reactions are known as serological reactions resulting into agglutination, precipitin formation, complement fixation, etc.
These antigens have the ability to stimulate within the host due to the formation of homologous substance called antibodies. The antibodies are specific and their function is to bind specific antigens responsible for their production and thereby inactivate or kill them. These antibodies are present in host serum, and are referred as immunoglobulins. The in vitro reaction of antigen and antibody is called serology and this provided the basis in learning diagnostics, therapeutics, and epidemiological implications. The serological reactions are agglutination, precipitin formation and complement fixation.
The specific antibodies called agglutinins are used in this reaction. These are formed in response to the introduction of particulate antigens into host tissues. The bacterial cells, redblood cells, and latex particles coated with antigens are particulate antigens. When the antigens combine with antibody, a three dimensional lattice complex forms, called agglutination. This agglutination reaction can be seen by naked eye.
The entry of soluble, non-particulate antigens into the host triggers it to produce specific antibodies. These antibodies in serum, form a complex with the specific homologus nonparticulate antigens results into visible precipitation.
The binding of certain serum proteins occurs in some antigen- antibody reactions. This is an invisible reaction and requires an indicator. The indicator consists of sensitised sheep RBC.