The Micronutrient Information Center is a public service of the Linus Pauling Institute and it has this to say about vitamin A:
Vitamin A and its metabolites play critical roles in both innate and adaptive immunity. In innate immunity, the skin and mucosal cells of the eye and respiratory, gastrointestinal, and genitourinary tracts function as a barrier against infections. Vitamin A helps to maintain the structural and functional integrity of these mucosal cells. Vitamin A is also important to the normal function of several types of immune cells important in the innate response, including natural killer (NK) cells, macrophages, and neutrophils. Moreover, vitamin A is needed for proper function of cells that mediate adaptive immunity, such as T and B cells; thus, vitamin A is necessary for the generation of antibody responses to specific antigens.
In the book, ‘Military Strategies for Sustainment of Nutrition and Immune Function in the Field (1999), chapter 12, the following is reported related to HIV and vitamin A deficiency:
A study involving HIV-seronegative injection drug users from inner city Baltimore showed that adults with vitamin A levels consistent with deficiency (serum vitamin A < 1.05 μmol/liter) had significantly lower CD4 counts than those without deficiency (Semba et al., 1993a). Studies in HIV-infected adults have shown a fairly consistent association between low vitamin A levels and low CD4 counts (Phuapradit et al., 1996; Semba et al., 1993a, b).
But Zinc is also essential for immunity and that is why Sally T. Vitamin A and Zinc is a good combination. Read what The Micronutrient Information Center has to say about zinc:
Zinc is critical for normal development and function of cells that mediate both innate and adaptive immunity. The cellular functions of zinc can be divided into three categories: (1) catalytic, (2) structural, and (3) regulatory. Because zinc is not stored in the body, regular dietary intake of the mineral is important in maintaining the integrity of the immune system. Thus, inadequate intake can lead to zinc deficiency and compromised immune responses. With respect to innate immunity, zinc deficiency impairs the complement system, cytotoxicity of natural killer cells, phagocytic activity of neutrophils and macrophages, and immune cell ability to generate oxidants that kill invading pathogens. Zinc deficiency also compromises adaptive immune function, including lymphocyte number and function. Even marginal zinc deficiency, which is more common than severe zinc deficiency, can suppress aspects of immunity. Zinc-deficient individuals are known to experience increased susceptibility to a variety of infectious agents.