Immune system: Antibody class differences

Each antibody-producing cell makes a single, unique type of antibody, which belongs to one of the 5 major classes – IgA, IgD, IgE, IgG, and IgM. Classes differ in their constant regions and their roles in the immune response. They bind their targets with varying degrees of strength, depending on the fit between the antibody and target.

During primary exposure to an antigen, the first classes produced are IgM and IgD, although IgD is rarely, if ever, secreted. IgM forms a pentamer, and its multiple binding sites allow it to attach to multiple antigens simultaneously. This property helps clump together pathogens, a process known as agglutination that enhances the immune response by making it easier for immune cells to identify and eliminate pathogens.

IgM antibodies act as receptors on the surface of B cells. When an antigen binds to these IgM receptors, it triggers B cell activation, leading to differentiation into antibody-secreting plasma cells. When a B cell receives the stimulus to initiate clonal selection, it undergoes class switching to IgG, IgA, or IgE; this final antibody is more specific for its pathogen and binds with higher affinity.

IgG is the most abundant antibody in the blood and lymph fluid and is the only antibody able to cross the placenta. The half-life of IgG antibodies can range from about 21 days to several months. IgA, which forms a dimer, is the most abundantly produced antibody class, but it is mostly found in glandular secretions such as tears, saliva, and mucous. It plays a major role in preventing respiratory and intestinal infections. Lastly, IgE is closely associated with responses to bigger invaders such as parasites. It is also responsible for allergies – when activated, it stimulates mast cells to release granules of histamine, which leads to symptoms such as itching and anaphylaxis.

Below are the antibody titers for a patient who has come into the doctor’s office with a severe skin infection.

Table 1 Antibody serum levels of patient A as compared with the average

Which statement accurately provides insight into Patient A’s condition, according to the information presented in the passage?

(Choice A)
Patient A’s elevated IgE levels indicate an allergy – this is a recent condition.
(Choice B)
Patient A has elevated IgG levels, meaning that this infection has just started.
(Choice C)
Patient A’s normal IgA levels indicate that patient A was previously exposed.
(Choice D)
Patient A’s normal levels of IgM suggest that this is not a new infection.

MCAT

Course: MCAT > Unit 2

Problem