7.2.2. Acquired Immunity

Acquired immunity, on the other hand is pathogen specific. It is characterised by memory. This means when our body encounters a pathogen for the first time it 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.

Acquired immunity specifically responds to particular pathogens (germs that can cause diseases). When the body first encounters a pathogen, it launches a primary immune response that is weaker and takes some time to develop. However, if the same pathogen attacks again, the body responds much faster and more vigorously. This is because the immune system 'remembers' the first attack, which enhances the response during subsequent encounters. This phenomenon is crucial in understanding how vaccinations work.

Think of acquired immunity like training for a sport. In your first practice, you may not perform very well because you are unfamiliar with the game. But after several practices, you start to remember the rules and improve. When you finally compete, you perform much better because your body has adapted to the game.

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 to produce them.

Lymphocytes are a type of white blood cell essential for the immune response. There are two main types involved in acquired immunity: B-lymphocytes and T-lymphocytes. B-lymphocytes respond to pathogens by producing antibodies, which are proteins that specifically target and neutralize foreign invaders. T-lymphocytes assist B-cells and are also crucial in destroying infected cells or regulating the immune response. This collaboration between B and T cells ensures a robust defense against pathogens.

Imagine a sports team during a match. B-lymphocytes are like the players who are actively attacking the opponent's goal (producing antibodies to neutralize pathogens), while T-lymphocytes are like the coaches who guide the team, helping them coordinate their actions and strategies effectively to win.

Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as H2L2. Different types of antibodies are produced in our body. IgA, IgM, IgE, IgG are some of them. Because these antibodies are found in the blood, the response is also called the humoral immune response.

Antibodies are specialized proteins crucial for identifying and neutralizing pathogens. They consist of four chains: two light chains and two heavy chains, forming a Y-shaped structure. This unique shape allows antibodies to bind specifically to antigens, which are substances that provoke an immune response. There are different classes of antibodies (e.g., IgA, IgM, IgE, IgG) that serve different roles in the immune response. The overall response involving antibodies in the blood is referred to as humoral immunity.

Think of antibodies as specialized keys that open specific locks (antigens). Just like each key fits perfectly to one lock, each antibody is designed to bind to a specific pathogen. The body's ability to produce various keys allows it to defend against multiple types of locks and threats.

The second type is called cell-mediated immune response or cell-mediated immunity (CMI). The T-lymphocytes mediate CMI.

Cell-mediated immunity involves T-lymphocytes, which play a critical role in recognizing and killing infected cells. Unlike B-lymphocytes, T-cells do not produce antibodies. Instead, they can directly attack and eliminate cells infected by viruses or other pathogens. This aspect of the immune response is essential for combating intracellular infections, where pathogens invade the host's own cells.

Imagine T-lymphocytes as specialized security guards in a building. If they detect that a room (cell) has been compromised by an intruder (pathogen), they can take action—either by quarantining the room or escorting the intruder out, thus preventing further access to the rest of the building.

The body is able to differentiate ‘self’ and ‘nonself’ and the cell-mediated immune response is responsible for the graft rejection.

One of the crucial features of acquired immunity is the ability to differentiate between 'self' (the body's own tissues) and 'nonself' (foreign tissues). This differentiation allows the immune system to recognize transplanted organs or tissues as foreign and potentially harmful, which can lead to rejection unless carefully matched. Understanding this concept is vital for successful organ transplants and immunosuppressive therapies.

Consider the body as a party where only invited guests (self-cells) are welcome. If an uninvited guest (foreign tissue) arrives, the hosts (immune cells) will not hesitate to remove them to keep the party secure. This analogy helps explain why organ transplants often require matching donor and recipient tissues to minimize rejection responsively.

--