T Lymphocytes: The Cellular Immune Response

0:25Phagocyte engulfs the pathogen

First, a reminder. An antigen is a molecule, usually a protein, on the surface of a cell or pathogen, and the immune system uses it to recognise what is foreign. The phagocyte has engulfed the bacterium. But here is the clever part. It does not simply destroy it and move on. It keeps pieces of the bacterium's antigens. Why bother? Because T cells cannot spot a pathogen floating freely. They only respond to antigens that are displayed on the surface of another cell.

So the phagocyte acts like a messenger, holding up a sample of the enemy for the T cells to inspect.

1:06Antigen presentation

The phagocyte moves those antigens to its own cell surface membrane, and displays them. A cell that does this is called an antigen-presenting cell. Remember the words, on the cell surface. The examiner wants that detail.

1:22Helper T cell receptor

Now meet the T lymphocytes. Each T cell has receptors on its surface. And each T cell has a different receptor, so there are lots of different T cells in the body. Most of them will never meet their match. But one helper T cell has a receptor with a shape that is complementary to the antigen. It binds to the antigen on the antigen-presenting cell.

That is the trigger. The helper T cell is now activated, and it divides by mitosis to make a clone of identical helper T cells. This is called clonal selection. The pathogen selects the one T cell that fits, and only that T cell is copied. That gives you many cells, all able to recognise the same antigen.

2:10Helper T cell activates three responses

Helper T cells are the managers of the immune response. They do not kill anything themselves. Instead, they release chemical signals called cytokines, and those signals stimulate other cells. There are three targets. Let's take them in turn.

2:26Cytotoxic T cells

Target one, cytotoxic T cells. The helper T cell stimulates them to divide and increase in number. Cytotoxic T cells then recognise infected body cells by the antigens on their surface, and destroy them. They do this by releasing substances that make holes in the cell's membrane. That is how they kill. Helper T cells coordinate. Cytotoxic T cells kill. This is why it is called the cellular response. The work is done by whole cells attacking, rather than by antibodies floating in the blood.

It is especially useful against viruses, which hide inside body cells where antibodies cannot reach.

3:09B cells stimulated

Target two, B cells. The helper T cell stimulates B cells to divide, and to secrete antibodies. We will cover that properly in the next lesson. Target three, phagocytes. The helper T cell stimulates them to engulf more pathogens. So here is the sneaky part. T cells do not make antibodies, but they are the reason B cells do.

3:35The whole flow

Now the answer, built by mark points. A phagocyte engulfs the pathogen and presents its antigens on its cell surface. That is one. A helper T cell with a complementary receptor binds to the antigen. That is two. The helper T cell stimulates cytotoxic T cells, which kill infected cells. That is three.

It stimulates B cells to divide and secrete antibodies. That is four. And it stimulates phagocytes to engulf more pathogens. That is five.

4:09Examiner trap and recap

The trap. Never say T cells produce antibodies. B cells do that. And do not mix up your two T cells. Helper T cells stimulate. Cytotoxic T cells kill. Recap. Antigens are presented on a cell surface, a helper T cell binds, and then it stimulates three other cells. Next lesson, we follow the B cell, and see how it makes antibodies.