The skin--a first level lymphoid organ?
D000818:Animals; D005075:Biological Evolution; D001717:Birds; D002060:Bursa of Fabricius / Q000276:immunology*; D008221:Lymphoid Tissue / Q000276:immunology; D012867:Skin / Q000276:immunology*; D013950:Thymus Gland / Q000276:immunology*