Dendritic cells are a form of antigen-presenting cell. Dendritic cells in epithelia are known as Langerhans cells, and those within germinal centers are called follicular dendritic cells (FDCs). The FDCs may become infected but not killed by HIV. They have cell surface Fc receptors that capture antibody-coated HIV virions through the Fc portion of the antibody. These virions attached to the FDCs can infect passing CD4+ lymphocytes. Dendritic cells elaborate type I interferons that up-regulate antiviral proteins in neighboring cells. B cells are a component of humoral immunity, and antibody to HIV does not serve a protective function, but allows serologic detection of infection. CD8+ cells are cytotoxic lymphocytes that lack the receptor necessary for infection by HIV. Because they survive selectively, the CD4+:CD8+ ratio is reversed so that it is typically less than 1 with advanced HIV infection. Innate lymphoid cells resemble NK cells, but shape further lymphoid reactions. Langhans giant cells are "committees" of activated macrophages that are part of a granulomatous response. Macrophages are a type of antigen- presenting cell that can become infected by HIV without destruction. Mast cells have surface-bound IgE, which can be cross-linked by antigens (allergens) to cause degranulation and release of vasoactive amines, such as histamine, as part of anaphylaxis with type I hypersensitivity.