Final answer:
The immunoglobulin most common in the alimentary canal is secretory IgA, which is present as a dimeric molecule and dominates the mucus secretions of the gastrointestinal tract. It protects mucous membranes by binding pathogens, and is also found in saliva, tears, and breast milk.
Step-by-step explanation:
The immunoglobulin that is most common in the alimentary canal is secretory IgA. Secretory IgA is specially adapted to protect mucous membranes by trapping pathogens in mucus, facilitating their elimination from the body. It predominates in mucus secretions, including those of the gastrointestinal tract, and is crucial in defending against infections in the alimentary canal.
Secretory IgA is typically found as a dimeric molecule, linked together by a J chain and with a secretory component that enables its mucosal transport. This form is essential for mucosal immunity, as it is more effective than monomeric IgA at neutralizing pathogens. Furthermore, secretory IgA is also present in other secretions such as saliva, tears, and breast milk, contributing to the immune defense beyond the alimentary canal.
It's important to note that while IgG is the most abundant antibody in the serum, it is IgA, especially secretory IgA, that is most abundant and effective in the mucus secretions of the body. Individuals with selective IgA deficiency are unable to produce secretory IgA and, as a result, may be more susceptible to infections in the lungs and gastrointestinal tract.