A lymphocyte is a reasonably common white blood cell with a very important function in acquired immunity. It can actually come in different forms, but sticks out because, in relation to the size of the cell, it has a simply enormous nucleus! This of course makes it handy when we want to get hold of some DNA from the blood.
Like the other cells of the blood, they are made in bone marrow, but they don't mature until they get into a lymphoid organ, where the body makes sure that they're doing the right thing, and eventually sends them off to do it!
The main types of lymphocyte are T cells and B cells, but another important lymphocyte is the natural killer cell.
Lymphocytes are normally a similar size to red blood cells, and they are less likely to change size; so, when looking at blood under the microscope, it can be useful to compare the size of the two to see if blood cells are too large or too small.
T-cells are so called because they mature in the thymus. They have a receptor on the surface unoriginally titled the T-cell receptor which is specific to each T-cell. They are cleverly manufactured so that pretty much every possible different structure exists, and they're there to recognise antigens that are presented by MHC molecules on both antigen presenting cells and normal cells. Lots of T-cells will go past, but when one that happens to have the right T-cell receptor recognises the antigen being presented, it becomes activated.
There are two very important types:
T-helper cells are, as the name suggests, there to help out. They have a protein on the surface called CD4, so they are sometimes called CD4+ T-lymphocytes, and when they are activated they produce a chemical called interleukin-2 (IL-2) to activate B-cells to produce a response. They need antigens to be presented on MHC Class II molecules in order to be activated.
Cytotoxic T-lymphocytes (CTL), as you might gather from the name, kill cells. They have a protein on the surface called CD8, so they are sometimes called CD8+ T-lymphocytes, and when they are activated they will kill the cell which activated them. These cells can recognise antigens if they are presented on MHC Class I molecules, which means they will be activated by antigens presented on any cell which is infected by a bacteria or virus.
It is a common misconception that the 'B' in 'B-cell' stands for bone marrow - because, like all blood cells, they are made in the bone marrow, and in fact they mature there too. In birds they mature in an organ called the bursa of Fabricius and it is from this that they obtain their 'B'.
When B cells are activated, they multiply and can develop into plasma cells and memory cells. Memory cells are there to remember the threat which first made an appearance, so that they can attack it if it comes in at a later date; there are a lot more memory cells than the original B cell from which they stemmed, so it becomes an incredibly useful supply.
Plasma cells are the ones which produce antibodies, which are proteins that can bind to antigens in order to inactivate them and prevent them causing damage.
T-cells also produce memory cells, although it is uncertain whether these arise when the original T-cell is activated, or if the active T-helper or cytotoxic T-lymphocyte becomes a memory T-cell when it's done its job. Either way, the memory T-cell has a similar function of waiting around until the specific antigen makes an appearance again, at which point the memory cells will be well placed to mount a specific and rapid response.
Not entirely surprisingly, the natural killer cell kills. It's there to kill. Indeed, it's rather a big fan of the whole 'killing' thing. It's the front-line soldier, targeting dangerous cells which pose a problem as soon as it's on the scene.
The natural killer cell is part of the body's innate immunity, as it does not respond to specific antigens, but rather will kill any cell that is has the characteristic features of a dangerous cell.
There are several things which cause a natural killer to think something's a bit wrong and decide to kill its target. One is a low concentration of MHC Class I molecules. Every nucleated cell contains these, so if there's not much there then it indicates something's wrong - and indeed, if it's infected by a virus then a cell may well have been prompted to produce less MHC Class I. A cell may also produce less if it is cancerous, since it is dividing quickly and has less time to produce such proteins. Natural killers, then, are effective cells at offering first defence against many of the body's problems, but they do not match up to the specific response offered, when possible, in acquired immunity.