Explain Acquired immunity.

Acquired immunity, on the other hand, is pathogen specific. It is characterised by memory. This means that our body when it encounters a pathogen for the first time produces a response called primary response which is of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response. This is ascribed to the fact that our body appears to have memory of the first encounter.

          The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood, i.e., $B-$ lymphocytes and $T-$ lymphocytes.The $B$ lymphocytes produce an army of proteins in response to pathogens into our blood to fight with them. These proteins are called antibodies.

The $T-$ cells themselves do not secrete antibodies but help $B$ cells produce them. Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as $H_2L_2$ Different types of antibodies are produced in our body. $IgA, IgM, IgE, IgG$ are some of them. A cartoon of an antibody is given in Figure. Because these antibodies are found in the blood, the response is also called as humoral immune response.

This is one of the two types of our acquired immune response -antibody mediated. The second type is called cell-mediated immune response or cell-mediated immunity $(CMI)$. The T-lymphocytes mediate $CMI$. Very often, when some human organs like heart, eye, liver, kidney fail to function satisfactorily, transplatation is the only remedy to enable the patient to live a normal life. Then a search begins -to find a suitable donor.

Grafts from just any source -an animal, another primate, or any human beings cannot be made since the grafts would be rejected sooner or later. Tissue matching, blood group matching are essential before undertaking any graft/transplant and even after this the patient has to take immuno-suppresants all his/her life. The body is able to differentiate ‘self’ and ‘nonself’ and the cell-mediated immune response is responsible for the graft rejection.