Explain the phenomenon of adaptive immunity with special reference to its properties, activation, clonal selection and its role in vaccination.

Explanation

Acquired immunity properties.
Acquired immunity is also called specific and adaptive immunity. It is mediated by lymphocytes or antibodies or both, which make the antigen harmless. It not only relieves the victim of infection, but also prevents its further attack in future due to formation of memory B and T-cells. Acquired immunity has two components.
1. AMIS (Antibody mediated immune system) or humoral immunity) consists of B-cells and antibodies.
2. CMIS (cell mediated immune system) consists of T-cells.
Helper T cells. They are numerous. These cells stimulate the B-cells to produce antibodies. They also stimulate the killer T cells to destroy the nonself cells. Their role is overall regulation of immunity. They do this function by forming a series of protein mediators, called lymphokines that act on other cells of the immune system as well as on bone marrow cells, (a) Cytotoxic T cells (= Killer Cells or K Cells). These cells directly attack the foreign cells. The cytotoxic T cells secrete a protein perforin which punctures the invader.s cell membrane. Water and ions flow into the nonself cell, which swells up and finally lyses. The cytotoxic T cells also destroy the cancer cells. The cytotoxic cells are responsible for cell mediated immunity, (b) Suppressor T Cells. They are capable of suppressing the functions of cytotoxic and helper T cells. They also inhibit the immune system from attacking the body.s own cells, (c) Memory T Cells. These cells remain in the lymphatic tissue (e.g., spleen, lymph nodes) and recognize original invading antigens, even years after the first encounter. These cells keep ready to attack as soon as the same pathogens infect the body again.
B Lymphocytes (=B-Cells). Certain cells of the bone marrow produce B lymphocytes. These cells mature in the bone marrow itself. The B cells produce specialised proteins called antibodies and, therefore, generate antibody mediated or humoral immunity.
The B lymphocytes give rise to: (a) Plasma Cells (Effector B cells). Some of the activated B cells enlarge, divide and differentiate into a clone of plasma cells. Although plasma cells live for only a few days, they secrete enormous amounts of antibody during this period. A few days after exposure to an antigen, a plasma cell secretes hundreds of millions of antibodies daily and secretion occurs for about 4 or 5 days until the plasma cell dies, (b) Memory B Cells. Some activated B cells do not differentiate into plasma cells but rather remain as memory cells. They have a longer life span. The memory cells remain dormant until activated once again by a new quantity of the same antigen.
Clonal Selection.
As we know, each B and T lymphocyte has a specific receptor on its surface. In B lymphocyte, this receptor is the antibody which is produced by that cell. When this receptor interacts with the antigenic determinant specific to it, the lymphocyte is activated and divides to form a clone of cells. These cells are also converted into effector cells (antigen producing B lymphocytes and cytotoxic T cells). This phenomenon is known as clonal selection, where all the cells in a givea T lymphocyte or B lymphocyte clone are derived from a single parent cell clone and show the same specificity for antigenic determinant. But some activated lymphocytes become memory cells which are long lived but do not produce antibodies or kill infected cells. As stated earlier memory cells remain dormant until activated again by the same antigen.