Aquired immunity is granted by the Innate immune system which takes place after the first onset of immunity mediated by the innate immune system.

Upon recognition of a foreign invader, the NK cells of the innate immune system responds by destroying them and further releases cytokines and chemokines which recruit other cells such as macrophages and granulocytes for a cooperative defensive mechanism which results in inflammation. Following innate immune responses, the T and B lymphocytes mediates an adaptive immune response which has the capability to recognize specific antigens and mediate a more effective mechanism to eliminate the foreign invader. This mechanism is highly adaptable due to somatic hypermutation (a process of accelerated somatic mutations), and V(D)J recombination (an irreversible genetic recombination of antigen receptor gene segments). This mechanism allows a small number of genes to generate a vast number of different antigen receptors, which are then uniquely expressed on each individual lymphocyte. Because the gene rearrangement leads to an irreversible change in the DNA of each cell, all of the progeny (offspring) of that cell will then inherit genes encoding the same receptor specificity, including the Memory B cells and Memory T cells that are the keys to long-lived specific immunity.

Activation of the Acquired Immune System
Unlike the innate immune system, the acquired immune system needs to have seen a substance before in order to attack it effectively. The reason for this is because the way that the acquired immune system attacks a target is very specific and has to have time to prepare the machinery required. All the things that the body wants to keep out have unique patterns on their surface that allow the cells of the acquired immune system to detect them. When the cells detect these patterns they know that they are not part of the body and so can mount an attack. Anything that the immune system can detect and attack is called an antigen. The activation of the acquired immune system initially requires the help of other cells. The cells of the acquired immune system are coated in 'receptors' that are highly specific molecules designed to recognise certain substances. These receptors are so specific however, that they can only act against it and nothing else. There are just so many immune cells in the blood, each with its own different receptor that means the body can be protected against different things. To speed up the process of activation, cells called macrophages (which means 'Big eater') help out. These macrophages are found in lots of places throughout the body and eat anything that they don't recognise. After they have eaten something, they can break it down into its basic proteins and present these to the immune cells, causing a better, more accurate and more damaging response than the macrophages alone would be capable of.

There are two types of immune responses,
T cell mediated immune response
B cell mediated immune response

T Cell Activation T cells cannot see foreign substances without assistance and require a complex system to help them work. They need the help of cells called Antigen Presenting Cells (APCs). These cells will eat the substance, be it a bacteria, virus-infected cell or toxin, break it down, and present part of it to the T cell so that it can mount a response. These APCs have a special type of molecule on their surface that means they can communicate with Helper T cells. Once a response is activated, lots and lots of T cells of different types are released into the blood stream and these cells are responsible for destruction of the foreign substance. Some of the different types of cells are spoken about below. Helper T cells Helper T cells are by far the most common of the T cells and make up more than three quarters of the T cell population. Helper T cells help the immune system in many different ways and serve as a major regulator of virtually all immune functions in the body. They mainly act through the release of substances that help control the other parts of the immune system. These substances (called lymphokines) stimulate the other types of T cells (talked about below) to grow and attack. They also help in the growth and maturation of B cells into their active form. In Acquired Immunodeficiency Syndrome (AIDS), there is a loss of helper T cells leaving the body open to infection. Also, due to the helper T cells influence of B cells, these are also quite inactive in cases where the T cells are damaged. Cytotoxic T cells require antigen presenting cells to be activated and their action is reliant on the presence of Helper T cells. Following activation by Helper T cells, these cytotoxic T cells prepare for the destruction of their target. Inside the cells, substances are formed which are incredibly dangerous to cells. They create a protein called 'perforin' that is named because it has the ability to 'perforate' the infected cell.