B Cells, Clonal Selection and Antibodies

0:28Selecting the right B cell

Your body has millions of different B lymphocytes. Each one carries receptors on its surface with one particular shape. When a bacterium arrives, its antigen is usually a protein on its surface, with its own shape. Think of a lock and key. Millions of keys, and only one fits this lock.

Only the B cell whose receptor is complementary to that antigen can bind to it. Binding is not quite enough on its own. The B cell also needs help from a helper T cell, which you met in the last lesson. That helper T cell releases signals that activate it. This is clonal selection. The one right B cell is selected from the crowd.

1:11Clonal expansion

Now the selected B cell divides by mitosis, again and again. That makes a large number of genetically identical cells, called a clone. This is clonal expansion. Each new cell is a copy, so every one of them has the same receptor, and the same ability to make the right antibody.

It matters because at the start only a few B cells fit, and you need an army.

1:37Plasma cells make antibodies

Most of the clone cells then differentiate into plasma cells. Plasma cells are antibody factories. They have lots of rough endoplasmic reticulum, because they are making protein, and they secrete large amounts of antibody into the blood and tissue fluid. Every antibody from one plasma cell has the same shape, matching the same antigen.

Some of the clone become memory cells instead. We will meet those in the next lesson, so just park that for now. Because the antibodies are released into body fluids, this is called the humoral response.

2:12Antibody structure

So what is an antibody? An antibody is a protein, made by plasma cells, that binds to a specific antigen. It has a Y shape, made of four polypeptide chains. Each chain has its own tertiary structure, and the four chains together give the antibody a quaternary structure.

The two arms of the Y each end in a variable region. That region has a particular tertiary structure, a shape that is complementary to one antigen. The stem and the rest of the molecule is the constant region, and it is the same in all antibodies of that type. So antibodies are specific because of the variable region, not the constant region.

2:56Antigen-antibody complex and agglutination

When an antibody binds to its antigen, they form an antigen-antibody complex. Each antibody has two binding sites, so one antibody can join two bacteria together. With lots of antibodies, the bacteria are clumped into a large mass. This clumping is called agglutination.

3:15Phagocytes clear the clump

Why does that help? A phagocyte can engulf many clumped bacteria at once. Free bacteria spread through the body. A clump is stuck together, so it is easier to find and to trap. So agglutination makes phagocytosis easier and faster.

The phagocytes then destroy the bacteria inside. The antibodies do not destroy the bacteria directly. They bind to them and clump them, and the phagocytes do the killing.

4:28Examiner trap and recap

The traps. Antibodies are made by plasma cells, not by antigens. Specificity comes from the variable region. And B cells never turn into T cells. Always say that agglutination makes phagocytosis easier. Recap. Select, clone, secrete, clump.

Next time, what happens when the same pathogen comes back, and why memory cells make the second response faster.