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MCAT

Unit 2: Lesson 3

Foundation 3: Organ Systems

Antibody class differences

Problem

Antibodies are protein molecules secreted by immune system cells that help the body fight invaders. Each antibody-producing cell makes a single, unique antibody which will be in one of the 5 major classes – IgA, IgD, IgE, IgG, and IgM. These classes are determined by the constant region of heavy chains of the antibody, and have different immunologic targets and mechanisms of action. They can also bind their targets with different degrees of strength, depending on the fit between the antibody and its target.
Figure 1. Basic antibody structure. Attribution: Username Je at uwo, CC-BY-SA 3.0.
The first classes produced are IgM and IgD. These proteins initially act as membrane-bound B-cell receptors (BCR). IgM forms a pentamer in order to better bind its target. IgD is rarely, if ever, secreted. When a B-cell receives the stimulus to initiate clonal selection, it also undergoes class switching and affinity maturation such that the final antibody is as specific for the pathogen as possible. IgG is the most abundant antibody in the blood and lymph fluid, and is the only antibody able to cross the placenta. IgA forms a dimer and is the most abundantly produced antibody class, but it is mostly found in secretions such as tears, saliva, and mucous. It plays a major role in preventing respiratory and intestinal infections. IgE is closely associated with a response to bigger invaders such as parasites. It is also responsible for allergies, when activated it can stimulate mast cells to release granules of histamine which lead to symptoms such as itching and anaphylaxis.
Below are the antibody titers (antibody concentration per unit blood) for a patient who has come in to the doctor’s office.
Table 1. Antibody serum levels of patient A as compared to the average.
Average serum level (mg/mL)Patient A
IgA2.12.3
IgD4.0 x 10, start superscript, minus, 2, end superscript4.1 x 10, start superscript, minus, 2, end superscript
IgE3.0 x 10, start superscript, minus, 5, end superscript3.3 x 10, start superscript, minus, 5, end superscript
IgG13.520.4
IgM1.51.6
How can one tell whether patient A’s condition is recent?
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