Why do the white blood cells allow foreign bodies like sperm to exist inside body cavity without any reaction?
White blood cells function is to fight against foreign bodies (from the text books).
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2 Answers
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Majority of white blood cells in general are present only within blood vessels, lymph vessels and lymph nodes. In addition various organs have locally adapted specialised immune cells. For example brain has microglia, Liver has kupfer cells, etc... Specialised immune cells are present along the mucosal linings which are named differently in different regions - Peyer's patch in the jejunum and ileum, tonsils in oral cavity, adenoids in the nasopharynx, etc...
As sperm do not enter blood vessels, only local mucosal immunity will act on them. There is a local immune response to the sperm in the vaginal mucosa. However, the immune response to the sperm is transient. Several hypotheses have been put forth to explain this:
- The seminal plasma suppresses the immune response. This is supported by the fact that sperm, if introduced without seminal plasma, do elicit an immune response--however this suppression seems to be inconsistent
- Insemination might activate a sequence that results in active immune tolerance to paternal antigens (antigens present in sperm)
For details see here:http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1782486/
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Studies show semen does mount a considerable immune response in the female mucosa, however:
Seminal plasma redirects the differentiation of human dendritic cells (DCs) toward a regulatory profile. By promoting a tolerogenic profile in DCs, seminal plasma might favor fertility, but might also compromise the capacity of the receptive partner to mount an effective immune response against sexually transmitted pathogens.
Semen promotes the differentiation of tolerogenic dendritic cells.
