Cells and Molecules of Immunity
Objective 9
Describe the cells and molecules which make up the main components of the immune response.
Cells of the Immune System
This image shows the cells of the immune system. Some of these you met already (e.g. the fibroblast in Unit 7) or will soon (the endothelial cell lining blood vessels in Unit 16).
In Objective 4, you met the granulocytes. They’re in the left-hand column.
- basophil
- eosinophil
- neutrophil
You also met the agranulocytes:
- macrophage
- lymphocytes
- B cell
- plasma cell, the active, antibody-secreting form of the B cell
- natural killer cell
- T cell
- B cell
We will also discuss two cells that have specialized roles in the immune system:
- dendritic cell
- mast cell
Also illustrated here are two cells that will play supporting roles but are not technically part of the immune system:
- endothelial cells, a squamous cell that lines the inner surface of all blood vessels
- fibroblasts, who we met in Unit 7 as the main cell type of connective tissue
Antigen-Presenting Cells
As we will see later, the immune system would rather deal with bite-size pieces of steak rather than trying to digest a whole side of beef. Chopping up the invaders into bite-size pieces is the job of antigen-presenting cells (APCs).
Like a parent making an airplane out of an unpleasant food on a fork, the antigen-presenting cells display pieces of the invaders in a way that makes them attractive to the T cells. This is called, of course, antigen presentation.
Macrophages are in a great position to act as antigen-presenting cells. As the garbage collectors of the body, they’re already patrolling the connective tissues and looking for things to eat and chop into pieces. If one of these pieces is “interesting” to the immune system, it will be presented to a T cell for its review and approval.
Dendritic cells, also derived from monocytes, act as APCs also. They visit sites of inflammation (“war zones”) and take “surveillance photos” of enemies. Then, they present these surveillance photos (antigen fragments) to the other cells of the immune system, particularly T cells, in a nearby lymph node.
B lymphocytes are called “naïve” when they have not yet encountered the antigen which they’re specific for. Once B lymphocytes are experienced, they can also act as APCs.
APCs use a specialized form of an eating process we first saw in Unit 4, phagocytosis (literally, “eating cell process”).
Phagocytosis involves a number of steps that are mostly logical if you remember phagocytosis means cell eating. Just think of the last time you walked into a kitchen when something tasty was being prepared. The smell led to you to the food, you reached out and brought the food to your mouth, chewed it up and swallowed it. Hopefully you didn’t show off your chewed food to family members or friends, but for the sake of the analogy, let’s pretend you did. As you shout “see food!” and open your mouth, you’re presenting what you ate. That’s antigen pr