Stella M. Hurtley, "Immunology: Taking A Peek,"
"Our bodies are covered by epithelial layers inside and out, which keeps the outside out and the inside in. How then can the immune system, which sits inside the wall of epithelial cells, sense potentially pathogenic antigens without making holes in this protective barrier? Kubo et al. show that during inflammation, epidermal Langerhans cells acquire external antigens by extending cellular protrusions, known as dendrites, through the tight seals between keratinocytes in the skin. Receptors on the tips of the dendrites bind to external antigens, which are then internalized and brought inward to the cell body for further processing. In order to maintain the seal despite breaching the tight junctions, the Langerhans cells form secondary junctions with the surrounding keratinocytes. This ability to screen incoming antigens provides an important first defense against attack."
Why should the targets of the Langerhans cells be limited to antigens? Presumably, the Langerhans cells don't "know" the molecules they grab and bring inside the body for identification are dangerous antigens. Why wouldn't they also grab for identification other organic molecules, such as pheromones and other aerosol molecular identifiers emitted by other persons, such as molecules emitted by women in ovulation or molecules emitted by men when they respond to women? Possibly they do this, implying that the meaning of "antigens" here is quite broad and not all "antigens" are pathological. Or that, if pathological, the antigens provide information useful for mate selection. If they would, this would make the skin a large sensory organ with a function like the nose.
Such an epidermal sensory organ would provide additional evidence that human beings are motivated to behavior by a suite of sensory organs whose biochemical stimulation of behavior is preconscious.