Topic: Immunology (Antibodies)
Immunoglobulins (antibodies) are categorized into five classes (IgG, IgA, IgM, IgE, IgD), each serving a unique role in the body's defense system.
Steps:
Understanding the Question: Which class of antibody is secreted onto surfaces like the gut, respiratory tract, and in body fluids like saliva?
Key Formulas and Approach: Focus on secretory immunity. One specific antibody forms dimers and is transported across epithelial cells.
Detailed Solution:
IgA is the predominant antibody found in mucosal secretions (tears, saliva, colostrum, and secretions from the genitourinary and digestive tracts).
It exists as a dimer (two units joined by a J-chain) in secretions, which makes it resistant to digestion by enzymes in those environments.
Its primary role is to prevent pathogens from attaching to and penetrating the epithelial lining.