We are all familiar with streptococcus: bacteria that cause throat inflammation usually resulting in a mild fever and antibiotic treatment, but may also lead to serious complications that can damage the heart. What prevents the bacteria from flourishing in the body and inflicting severe damage? Hint – it is not only antibiotics.

Inside our body there are trillions of white blood cells with the sole purpose of locating and eliminating microscopic factors that pose a threat on the body. The following video presents one part of the sophisticated protective system of the body. We can observe here how the antibody-secreting B cells are activated, and how the appropriate cells are selected and sent to fight the threat

The video was produced by Drew Berry as a part of the WEHI-TV project. We would like to thank Drew Berry for his help. For additional information on activation of the immune response you can watch the following videos: Activation of the Immune Response by TLR Receptors,  Dendritic Cells and Activation of the Innate Immune System


The “Clonal Selection” model, discovered by Sir Frank Macfarlane, which also won him the Nobel Prize in Medicine is the principle behind the adaptive immune response. The principle itself is quite simple: there are a wide variety of cells, each of which recognizes one specific structure. When a foreign intruder enters the body, and one of these cells recognizes a structure upon it, that cell will be activated.

Following this activation, the cell divides into many copies – clones – and differentiates into plasma cells, which are active cells that secrete antibodies, and also into memory cells, which are the “backup” cells preserving the immunological memory

After a few days, a huge army of B cells, which are ready to fight the bacteria, already exists, and they do this by secreting antibodies. Antibodies are proteinous structures that are able to recognize the structure discovered by the B cell that secreted them. As the antibody interacts with such a structure, it immediately binds to it, thus signalling to the phagocytes and the complement system to eliminate the intruder.

The clonal selection model is not limited to B cells, but also works in T cells, which are responsible for assisting the activity of the immune system and for killing infected cells of the body. Clonal selection also occurs in the development process of B and T cells, as cells know how to recognize receptors that require a reaction and not to recognize other structures that naturally exist in our body (i.e. so they do not attack healthy cells). This is actually a form of evolution of a diverse population, in which one individual prospers more than the others due to an environmentally-favorable trait (i.e. recognition or lack of recognition of certain structures).


Erez Garty
Department of Biological Chemistry
Weizmann Institute of Science

Article translated from Hebrew by Elee Shimshoni, PhD student at the Weizmann Institute of Science.

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