IgA, a predominant immunoglobulin in mucosal tissues including the gastrointestinal tract, is vital in maintaining the mucosal barrier and preventing the adherence and invasion of pathogens. A deficiency in Secretory IgA would result in recurrent gastrointestinal infections, explaining the patient's diarrhea, abdominal pain, and weight loss.

Increased IgE levels are associated with allergic response and atopy, not recurrent infections or mucosal ulceration.

IgG deficiency would lead to recurrent systemic and respiratory infections but seldom presents with gastrointestinal symptoms.

Overproduction of IgM is associated with Waldenstrom's macroglobulinemia and can cause hyperviscosity syndrome, not chronic gastrointestinal symptoms.