What happens when a foreign substance enters your body, such as a virus or bacterium you pick up going about your day? Any molecule that elicits the adaptive immune response is called an antigen. Antigens are often molecules that protrude from the surface of viruses or from foreign cells such as bacteria. They may also be soluble molecules such as toxins. We'll use this simple representation of an antigen as we move forward. Antigens that enter the body fluids may be swept into the lymph and eventually to the lymph nodes where diverse pools of white blood cells, or lymphocytes, are located, but we'll focus our attention on a specific type of lymphocyte, the B cell. B cells that do not have matching antigen receptors will not bind this specific antigen. Recognition of an invader begins when an antigen binds with a B cell that has a corresponding antigen receptor. The B cell activated by antigen divides, forming identical clones specialized against this one antigen. This population of identical cells is called a clone, and the process of producing thousands of identical cells is known as clonal selection. T cells go through a similar process, too. Some B cells differentiate into effector cells that combat infection immediately. Effector B cells are known as plasma cells. Plasma cells produce Y-shaped proteins called antibodies, and each cell secretes as many as 2,000 copies of antibodies per second. Structure-function is a core theme in biology. So what cellular structure would you predict to be in abundance in plasma cells that function to synthesize large quantities of secreted proteins, that is, antibodies? Under a microscope the rough endoplasmic reticulum is prominent in plasma cells. The rough endoplasmic reticulum is the site of assembly and processing for proteins being shipped out of a cell. Once antibodies are secreted into the blood and lymph, what do they do? They bind to the specific antigen that triggered their production. This interaction is identical to the antigen-receptor binding we saw on the parent B cell which first recognized the antigen. Antibody binding enhances the uptake of antigen by a type of white blood cell called a macrophage, or big eater. Antigens are then destroyed inside macrophages. Let's think again about structure and function. What cellular structure would you predict to be in abundance in macrophages that function in antigen destruction? Macrophages have an abundance of lysosomes, the site where digestion and degradation occurs within a cell. Before we leave our discussion of antibodies, let's consider the timing. How long does it take for antibodies to be made after you have been exposed to an antigen for the first time? Say, after someone sneezes on you. After first exposure to an antigen, measurable levels of antibodies are not seen until about a week after exposure. And peak production is reached another one to two weeks later. The antibody level declines steadily over the next few weeks. So, if you were sick, you'd likely begin feeling better once enough antibodies were present to aid your macrophages. Let's go back to our clone. Only some of the cells became plasma cells. What becomes of others? Other cells differentiate into memory cells that remain in the lymph nodes waiting to be activated by a second exposure to the antigen. If a second exposure does come, the immune system is ready to produce a quicker and stronger defense because memory cells rapidly differentiate into a great quantity of plasma cells. As a result, antibody level is very high within days after a second exposure. Now that we have discussed both plasma cells and memory cells, test your understanding by thinking about which cell type provides long-term immunity after a vaccination. Long-term protection is provided by memory cells. A vaccination is a harmless or weakened version of an antigen and serves as a body's first exposure. Clonal selection ensues and memory cells are produced. These memory cells can last for decades, ready to respond if the natural antigen is encountered later in life. Let's review the cell types with a few helpful comparisons. Whereas plasma cells are highly effective at combating an existing infection, memory cells help activate the immune system upon subsequent infection. And plasma cells die off within four to five days, while memory cells can last for decades. Let's end with the big picture. Our immune system responds to an initial infection and helps us recover from illness, but it also keeps a memory of that infection, responding even more efficiently if the invader returns.