It is not that easy. Let me try.
"常见的解释是不是摄入体内的蛋白不能被充分消化吸收，就
从消化系统进入到体内其他部分，造成免疫系统认为有异类进入，进行攻击？" That
is an acceptable explanation. But not exactly the proteins we ate and via
digestive tract. But in general, it is a cross reaction, which means that
some antigens in our cells somehow similar to the stuff we are exposed to
like pollen, proteins, etc, our body of course will fight against those
exterior stuff, but because the similarity, unfortunately, those self cells
are attacked too.
"一般西医处理这类疾病的处方药都是激素类的，请问其中最主要的原理是什么？降低免
疫系统的敏感程度或者说是反应阙值？". That is also an extremely acceptable
explanation. Steroids suppress immune response through many pathways. But
for eczema or other allergic reactions, steroids inhibit T cell activation/
proliferation and cytokine production, and those T cells and cytokines are
responsible for those reactions. I assume you know what T cell and cytokines
are. If not, then think this way. T cells are the immune cells responsible
for this allergic reaction (not very accurate, but it is enough). Cytokines
are the products those T cells and other immune cells make to exert their
functions.
Hopefully this helps in a layman's words.
