BIOLOGY OF IMMUNE RESPONSE
DYNAMICS OF ANTIGEN ELIMINATION The basic objective of defense mechanisms of body is to eliminate antigen. The injection of an antigen into an animal may initiate several important changes. For intravenously injected antigen, three phases of antigen removal are easily detected: Phase of Equilibration First phase is of equilibration which lasts for 10 to 20 minutes if particulate antigens are used and a longer time if antigen is soluble. Phase of Non-immune Catabolic Degradation The second phase lasts for 4-7 days during which catabolic degradation and removal of antigen takes place. Phase of Immune Catabolism The third phase is of immune catabolism which is the result of combination of newly formed antibody molecules against the antigen, enhancing phagocytic engulfment, digestion and removal of antigen. The absolute removal of all antigen from an immunised animal can take many months or years. IMMUNE RESPONSE Primary Immune Response The primary immune response is defined as the host's first exposure to a specific antigen which results in appearance of specific serum antibodies. In the primary response, after the first injection of antigen there is a lag of several days (latent or inductive period) before antibody appears. Factors that influence primary antibody response are: . Chemical and physical nature of antigen . Use of adjuvants . Dosage of antigen . Frequency of antigen exposure . Route of antigen exposure . Genetic make up of host receiving the antigen. On stimulation, a B cell may produce antibody within 20 minutes but since very few cells produce antibody in beginning, it cannot be measured. Usually measurable antibody appears between the 5th and 10th day. The antibody secreting cells gradually disappear from the system because of influence of regulatory cells, their cytokines and antibodies. The memory cells, however, persist in lymphoid tissues. The memory cells do not participate in the initial response, but they retain their ability to recognise a particular antigen. Secondary Immune Response In this type, if the same animal is subsequently exposed to same antigen, there occurs a sharp drop in circulating antibody because it complexes with the newly injected antigen. After two or three days, a marked increase in antibody level becomes evident. This increase continues for several days and ultimately results in level of antibody which is higher than what is seen in primary response. This response is also often called as memory, or booster response or secondary immune response.
USES OF PRIMARY AND SECONDARY IMMUNE RESPONSES Primary and secondary responses may occur during microbial infections. When we recover from an infection without the help of antibiotics, it is usually because of the primary response. If, at a later time, we contact the same microorganism, the secondary response can be so swift that the microorganisms are quickly destroyed and no clinical feature of the disease is exhibited. Secondary immune response also provides basis for immunization against certain diseases. On administering initial immunization, the immune system gets primed. Should the antigenic stimulus be encountered again as the infecting microorganism or booster dose of vaccine, the body experiences the secondary response. A booster dose literally boosts the antibody titre to a higher level.