This activity is a simple model of the adaptive immune response, one part of the human body’s immune system response. While this is not the first step in a real immune response, it is an important one that is unique to humans and higher vertebrates, and allows for the body to target specific pathogens and remember them in preparation for future contact.
Pathogens can invade your body through breaks in the skin, or through mucous membranes in your eyes, nose, and mouth, creating internal infections. While bacteria often grow in the fluids between your cells, and can reproduce and spread through the body via the bloodstream, viruses have a different strategy. Viruses cannot reproduce on their own, so they insert their genetic material into your cells and use them as virus-making factories. The newly copied viruses then exit the cells and spread throughout the body.
In response, the body’s immune system launches a cascade of complex processes that end up with the antigen from the outside invader binding with a matching antibody. This joining takes place in the lymph nodes, on the surface of a specialized immune cell called a B cell. Because there are only a few B cells with antibodies that match any given antigen, the first contact with a specific antigen initiates a response that might take several days to become effective.
Once the match takes place, the B cells divide rapidly. Some become antibody-making factories called plasma cells, and some become memory cells, which retain the “memory” of that particular antigen for the future.
Plasma cells produce and release millions of antibodies into the bloodstream and lymphatic system. Those antibodies seek out and bind to specific antigens, disarming them, and stopping further spread of the pathogen. As you may have noticed in the activity, the fit isn’t always perfect, but in the body, it continues to improve as the B cells make more and more antibodies.
Because the unique Y-shape of the antibody creates two binding sites for antigens, multiple antigens and antibodies can clump together, creating globs of cells called agglutinations. These agglutinations attract phagocytes that find, ingest, and digest them, eliminating the dangerous pathogen and infected cells from the body. This process of antibody production and “cleaning”—represented in the activity by the plastic bag “gobbling up” the globs of material—continues for a few days until the pathogen is removed.
This activity matches just five kinds of antibodies and five kinds of antigens. In reality, there are millions of different kinds of each. Animals with adaptive immune responses have evolved the ability to not only target specific pathogens, but also to create memory cells that remember the pathogens they’ve been exposed to. When a familiar pathogen reenters the body, the immune system is prepared, the antibody launch is rapid and profuse, and the pathogen is often quickly eradicated. We call this “having immunity.”