B cell activation occurs in the secondary lymphoid organs (SLOs), such as the spleen and lymph nodes. After B cells mature in the bone marrow, they migrate through the blood to SLOs, which receive a constant supply of antigen through circulating lymph. At the SLO, B cell activation begins when the B cell binds to an antigen via its BCR. Although the events taking place immediately after activation have yet to be completely determined, it is believed that B cells are activated in accordance with the kinetic segregation model, initially determined in T lymphocytes. This model denotes that before antigen stimulation, receptors diffuse through the membrane coming into contact with Lck and CD45 in equal frequency, rendering a net equilibrium of phosphorylation and non-phosphorylation. It is only when the cell comes in contact with an antigen presenting cell that the larger CD45 is displaced due to the close distance between the two membranes. This allows for net phosphorylation of the BCR and the initiation of the signal transduction pathway. Of the three B cell subsets, FO B cells preferentially undergo T cell-dependent activation while MZ B cells and B1 B cells preferentially undergo T cell-independent activation.
B cells, unlike the other two classes of lymphocytes, T cells and natural killer cells , express B cell receptors (BCRs) on their cell membrane .  BCRs allow the B cell to bind to a specific antigen , against which it will initiate an antibody response. 
Vertebrates inevitably die of infection if they are unable to make antibodies. Antibodies defend us against infection by binding to viruses and microbial toxins, thereby inactivating them (see Figure 24-2 ). The binding of antibodies to invading pathogens also recruits various types of white blood cells and a system of blood proteins, collectively called complement (discussed in Chapter 25). The white blood cells and activated complement components work together to attack the invaders.
The white town of Oia.
Oia is the northernmost and perhaps the most romantic town on the island of Santorini. The white town literally hangs over the caldera. At every turn you can see old mills, narrow streets, terraced houses, stairs and cul-de-sacs, domes, arches and small hotels. In 1956, the town was partially destroyed by the earthquake, but it was quickly restored. Many hotels in Oia have no more than 5-6 rooms.
It is believed that Oia has the best and the most expensive restaurants, shops, as well as the most impressive view of the sunset. Thousands of people come to watch it every evening. The world`s most beautiful and romantic sunsets, which seem to exist only in Oia, are the most important thing here apart from the volcanoes glowing in the bay. You can watch the famous sunsets of Oia only from the western part of the resort, which overlooks Amoudi Bay. The eastern part is also very picturesque: it offers a wonderful view of Armeni Bay, but there are no such sunsets as in the west.
Traffic is forbidden in Oia, so nothing will prevent you from enjoying a walk through the streets, the embankment, shops, cafes and restaurants. But there is one place in Oia, which you can reach either by car or coming down the steps (235 steps). This is the old fishing port of Amoudi. Amoudi will immediately seem to be suspiciously familiar to you, and it`s not surprising – you can see it in many films.
Building norms in Oia are observed in the strictest manner; therefore, even new houses are built according to the Cycladic architecture style: rounded forms, white, blue and ochre tones, narrow stoned streets, stairs leading to patios in front of houses carved into rocks.
Patriarchal beauty of Oia is not violated by modern technologies, because thanks to its patrons, power lines and telephone lines of the town are laid underground. And this is the only place on the island where water is desalinated using a special unit, also received by the town as a gift.
Today Oia is a popular tourist center with a well-developed infrastructure. This town seems to be created for long walks and is perfect for lovers of quiet rest, and the magnificent panoramic views of the island and its giant caldera will leave a lot of unforgettable impressions.