Our body's immune system is responsible for preventing our organism's infection from foreign ones. Any such foreign particle that may trigger an immune response is known as an antigen. What's really impressive about our immune system is that it is capable of making the distinction between self cells and non-self antigens, a property which is necessary for its optimum functioning. However, there are cases where this "recognition ability" is malfunctioning resulting in a number of diseases.In response to an infection by a foreign antigen, our body is going to produce a large number of blood proteins, known as antibodies, which are going to help fight the infection. However, their production is a rather complex process divided into smaller steps taking place one after the other. First of all, when a pathogen, for example, invades our body, it is going to be ingested (engulfed) by a specific type of cells known as macrophages. After being ingested, the macrophages have the ability to display, on their own cell membrane, the pathogen's antigens so that they can be readily recognizable by the other cells involved in the process. Afterwards, another type of white blood cells, called helper T-cells, are going to get involved. These cells possess a specific receptor on their own cell membrane which can recognize and bind to the antigens displayed by the macrophage. This binding will result in the activation of the helper T-cell. However, it is also important to mention the fact that there are thousands of different types of T-cells in one's body, which will display different types of antigen receptors. Thus, a specific pathogen with specific antigens will activate a specific type of T-cells in the organism.One this has been done, the activated T-cells are going to travel inside our body so that to bind to and activate another type of white blood cells, named B-cells. Again, only a specific fraction of B-cells will get activated (only those that possess a matching receptor). B-cells are activated by T-cells through both direct binding, as well as through means of signalling. Once activated and matured, B-cells are going to start releasing large numbers of antibodies so that to fight the infection. More specifically, the B-cells will start dividing, after activation, resulting in a large clone of genetically identical cells, all producing the same type of antibody. This process is known as clonal selection. Out of this large clone, most of the cells will remain as active B-cells (plasma cells) producing the needed antibody. However, a small number of cells out of this clone will turn into memory cells. Memory cells, unlike the plasma cells, will not be lost after the infection has been eliminated, but instead they will remain inactive in the body until the same pathogen strikes again. In this case, if that happens, the memory cells will rapidly become active and start producing the required antibody straight away. This is an example of how immunity against a particular virus, for example, is acquired.Finally, it is important to mention that antibodies, after being released, can help fight the infection through multiple ways which can either occur separately or simultaneously, for more rapid results, depending on the type of antibody produced, as well as on the severity of the infection. The most common way through which they aid in the destruction of the invading particle is a process called opsonization. In that case, the antibody will render the foreign molecule more recognizable to macrophages and therefore more susceptible to engulfment. A way of doing that can be by directly linking these two together.