As stated in lesson 4, adaptive immune responses to foreign antigens are made up of cell-mediated and humoral processes. The lymphocytes that carry out these processes (T cells for cell-mediated and B-cells for humoral) have fundamental properties that are essential for proper immune system function.
And here they are:
1. Specificity and Diversity – Individual lymphocytes have receptors on their surfaces that are specific to individual antigens. In fact, they are not only specific to the antigen, but a single, subtile difference in an antigen–a characteristic complex protein, polysaccharide, or other macromolecule–something we call the antigen’s epitope. Specific lymphocytes recognize specific epitopes, and the huge number of lymphocytes with different receptors gives the immune system power to handle a diverse population of foreign antigens.
2. Clonal Expansion – After the lymphocyte encounters an antigen, it will not only express the receptors for the antigen, but will make copies of itself. These lymphocyte copies will have identical receptors for that antigen. The copies are called clones. And the more clones the body has, the better fight it will have against infection with that antigen.
3. Memory – When a naive (have never “seen” an antigen) lymphocyte first encounters an antigen, the lymphocyte will respond, produce clones of itself, and begin the attack. And then, the now mature lymphocytes (memory cells) will store a long-lived memory of this antigen. Should the antigen enter the body again, the mature lymphocytes will evoke a secondary immune response, which is much faster and larger (due to clonal expansion).
4. Specialization – This is a term used to describe the fact that the immune system will react differently to each different antigen. Some microbes will elicit a strong humoral immunity, some a strong cell-mediated immunity, others will evoke both. And even within those different responses, different immune cells may be at play. The body has a special and unique way to fend off all sorts of intruders.
5. Contraction and Homeostasis – Once an infection has cleared the body, the body’s immune system goes back to homeostasis (stability) and some of the lymphocytes that are no longer being stimulated by antigen are not needed and begin to die by apoptosis (programmed cell death), or what’s called contraction.
6. Nonreactivity to self – Lymphocytes recognize antigens as something that’s not part of the body (nonself), but what’s truly amazing is that they are able to recognize and not harm the cells that are part of the body. These “self” cell antigens are tolerated by lymphocytes and the tolerance is even maintained by lymphocytes. Meaning, lymphocytes are able to recognize if other lymphocytes have built receptors to self cells and have begun attacking the body. All lymphocytes that wish to attack the body are eliminated. Should this function not happen, it can lead to autoimmune disorders–when the immune system’s cells attack its self cells.
Each one of these features is essential for normal immune system function in defense against microbial antigens. Isn’t the immune system amazing? I hope you’re getting interested…
- Abbas, Abul K., Andrew H. Lichtman, and Shiv Pillai. Cellular and Molecular Immunology. Philadelphia: Saunders Elsevier, Inc., 2010.