Jane, 36 and Moses, 44, have been married for 17 years now and have five children. Until recently, they did not know Moses was HIV-positive. When Moses was tested for HIV at a health counseling and testing centre, he did not believe his results. He said before marriage, he had never had sex.He lamented that may be his wife could have infected him. However, when Moses and his wife tested together, she turned out to be HIV-negative. This situation where one partner is infected with HIV and the other one is not is called HIV discordance.There are also circumstances where a man is HIV negative after repetitive tests yet the wife died of HIV. He marries another woman who is HIV-positive and does not get infected.It is worth to understand that discordance is not related to blood groups, there is no particular blood group which protects people from being infected with HIV.Yet is possible some people to resist being infected with HIV despite being exposed to the virus over a long period.However, it should be noted that people in discordant relationships are at a higher risk of developing HIV than the general population. Such people are advised to use condoms correctly and consistently whenever they have sex instead of thinking they are immune to HIV. This is because they can easily become infected with HIV at any time.People in sexual relationships should go for HIV tests with their sexual partners. They should not imagine they are of the same status as their partners.Vaccine development is still under investigation to prove the actual causes of protective immunity against a targeted disease.However, with HIV an understanding of protective immunity is still lacking, hampering the development of a vaccine and other drugs to combat the virus. An example to support this evidence was a group of some researchers who carried out investigations over 10 years ago to track a group of commercial sex workers in Kenya who did not become infected with HIV despite repeatedly having sex without condoms.The major aim of the study was to investigate and analyse reasons why some people exposed to HIV virus did not get infected, whereas others would easily get infected so quickly when exposed to any HIV causative agent.This information is very important because if scientists can understand what constitutes and results in protective immunity against HIV, they may be able to replicate it through vaccines. Scientists proved that the difference in infection with HIV amongst the population is linked with the presence or absence of CCR5 and fusin (also known as CXCR4) receptors. These two new proteins found on immune cells, play a key role in understanding how HIV infects cells. Though this discovery may not have immediate impact on people with HIV, it may lead to important advances in HIV treatment, prevention and research in the future.People without CCR5 and CXCR4 receptors hardly get infected with HIV even after a long exposure. Most of the sexual commercial workers who were not found to be infected with the disease after long periods of exposure to men with HIV were found to have no receptors.CCR5 is present on a broad range of cells that can be infected by HIV. CXCR4, on the other hand, is primarily found on CD4+ cells and only appears to serve as a doorway for certain types of HIV.CCR5 appears to be important for NSI strains of HIV (the strains most common in early disease), while CXCR4 appears to be more important for SI strains (a more aggressive strain seen in some people with more aggressive disease).NSI (non-syncitium inducing) strains of HIV are the most common sexually transmitted form of the virus. This type of HIV preferentially infects macrophages (often found in the skin and mucous membrane) rather than T-cells.When HIV is transmitted sexually, it first establishes itself as an M-tropic virus, later developing into T-cell-tropic viruses in some people. These T-tropic strains that prefer to infect T-cells are SI (syncitium inducing) viruses and may become more prevalent during later stages of the disease.The SI strain of HIV is more aggressive and its prevalence correlates with more rapid disease progression.Additionally, anti-HIV drugs generally have less activity against SI strains of HIV. The most obvious difference between someone with an NSI versus an SI strain, however, is that people with an SI strain experience more rapid decline in CD4+ counts, as the SI virus preferentially infects and destroys these cells.Also, people with the SI strain tend to have at least a 3 to 5 fold increase in the rate of disease progression.