Page 170 - 84 human physiolofy part-1

Chapter 9

autoimmune disease). Sometimes we develop an immune response to a harmless antigen, such as
pollen or cat dander (this is an allergic response).

Lymphocytes

Specific immunity is dependent upon two types of lymphocytes, the B cells and the T cells. Their
names are based on where in the body they mature. B cells mature in the bone marrow, and T cells
mature in the thymus gland. In comparison, both B and T cells can recognize and target antigen-bearing
cells, although they go about this in different ways. B and T cell lymphocytes are capable of
recognizing an antigen because they have specific receptor molecules on their surface which exactly fit
individual antigens (like a lock and a key). Any B or T cell can only respond to one type of antigen.
The body does not know ahead of time which antigens it will encounter, but rather makes receptor sites
for a huge number of possible antigens. It is estimated that for the million or so antigens we encounter
in our lifetime we have an equal number of specific lymphocytes for each possible antigen.

B Cells Produce Antibodies

B cell lymphocytes are responsible for antibody-mediated immunity (humoral immunity). They
produce antibodies, which are proteins that bind with and neutralize specific antigens. Antibodies do
not directly kill bacteria, but mark them for destruction. When antibodies bind to viruses they can
prevent the viruses from infecting cells. When antibodies bind to toxins they can neutralize the toxin
(why we get immunized against the tetanus toxin). Humoral immunity works best fighting against
target viruses, bacteria, and foreign molecules that are soluble in blood and lymph before the bacteria
or viruses have entered into cells (extracellular bacteria and extracellular viruses).

B cells produce two different types of cells:

• plasma cells
• memory cells

Plasma cells

As B cells mature during embryonic development, they develop surface receptors that allow them
to recognize specific antigens. Then they travel in the bloodstream, distributing throughout the lymph
nodes, spleen, and tonsils. Once B cells reach their destination, they remain inactive until they
encounter a foreign cell with an antigen that matches their particular receptor site (most B cells remain
inactive for your entire life). The foreign antigen can be presented to the B cell directly, but usually
macrophages and T cell lymphocytes (helper T cells) interact with B cells as Antigen Presenting Cells
to bring about antibody production. Upon such an encounter, the B cell's receptors will bind to the
antigen. The appropriate B cell is turned on or stimulated. It then grows bigger, and rapidly multiplies
into a large homogenous group (clone). Most of these cells are plasma cells, which actively secrete
antibody that will bind with the original stimulating antigen . While most of the B cells remain in the
lymphatic system, the antibodies are secreted into the lymph fluid which then enters into the blood
plasma to circulate throughout the body. Although the clone cells only live a few days, their antibodies
remain and circulate in the blood and lymph, gradually decreasing in number.

Antibody Structure and Function

170 | Human Physiology