Explain Acquired immunity.

(N/A) Acquired immunity is pathogen-specific. It is characterized by memory.
When our body encounters a pathogen for the first time,it produces a response called the primary response,which is of low intensity.
Subsequent encounters with the same pathogen elicit a highly intensified secondary or anamnestic response. This is due to the fact that our body retains a memory of the first encounter.
The primary and secondary immune responses are carried out by two special types of lymphocytes in our blood: $B$-lymphocytes and $T$-lymphocytes.
$B$-lymphocytes produce an army of proteins in response to pathogens to fight them; these proteins are called antibodies.
$T$-cells do not secrete antibodies themselves but help $B$-cells produce them.
Each antibody molecule has four peptide chains: two small 'light chains' and two longer 'heavy chains',represented as $H_2L_2$.
Different types of antibodies are produced in our body,such as $IgA, IgM, IgE,$ and $IgG$.
Because these antibodies are found in the blood,this response is called the humoral immune response.
The second type of acquired immune response is the cell-mediated immune response or cell-mediated immunity $(CMI)$,which is mediated by $T$-lymphocytes.
$CMI$ is responsible for graft rejection in organ transplants,as the body can differentiate between 'self' and 'non-self' cells.