Page 147 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

2- IgG

1. Structure- All IgG's are monomers.

2. Properties

a) IgG is the major Ig in serum (80-85% of serum)

b) The most abundant circulating class produced during the secondary

response.

c) IgG antibodies provide protection by:

- neutralization,

- agglutination and precipitation, opsonization

- complement activation

- antibody-dependent cellular cytotoxicity

d) IgG is the only class of Ig that crosses the placenta.

• Maternal IgG protects the developing fetus and also the newborn against

infections, because of the relatively long half-life of this class.

• The protection provided wanes after about 3 to 6 months, but by this time the

infant has begun to produce its own protective antibodies.

e) IgG is also present in colostrum, the first breast milk produced after birth. The

intestinal tract of newborns can absorb this antibody

3- IgA

1. Structure- Serum IgA is a monomer but IgA found in secretions is a dimer.

2. Properties

a) IgA is the 2nd most common serum Ig.

b) The secreted form is important in mucosal immunity and is found on the mucous

membranes that line the gastrointestinal, genitourinary, and respiratory tracts and

in secretions (tears, saliva, breast milk).

General Microbiology & Immunology (PM 401) theoretical handbook Page 136