Final answer:
The transport of secreted IgA by the poly-Ig receptor across the mucosa epithelia is called transcytosis, which is essential for mucosal immunity by enabling the antibodies to neutralize pathogens.
Step-by-step explanation:
The transport of secreted IgA by the poly-Ig receptor across the mucosa epithelia is known as transcytosis. This process allows IgA antibodies to move from the inside of bodily tissues through the mucosal epithelial cells to the outside, such as into the gut lumen or other mucosal surfaces. This mechanism is crucial for mucosal immunity, enabling the antibodies to neutralize pathogens by preventing their attachment to the mucosal surfaces without causing inflammation.
The options provided in the question refer to other immunological functions. For example, complement fixation involves the antibody-mediated activation of the complement system, leading to pathogen opsonization and lysis. Neutralization refers to the process by which antibodies block pathogen attachment to host cells. Activation of macrophages and mast cells are further immune defense mechanisms that are not directly related to the transcytosis of IgA via the poly-Ig receptor.