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HIV Pathophysiology (1/3) - Overview

HIV almost always refers to HIV-1, the more virulent and infective type. The human immunodeficiency virus, or HIV, targets cells of the immune system. When the immune system starts to fail, this is termed immunodeficiency. In the final stage of the disease’s progression, a patient is said to have acquired immunodeficiency syndrome, or AIDS. AIDS is not the virus, but rather a set of symptoms that result from the virus. It is defined as either having a T-cell count below 200 cells per mL of blood, or as having HIV and at least one opportunistic infection associated with AIDS, regardless of T-cell count. An opportunistic infection is an infection caused by pathogens that would normally not be a problem, but which take advantage of an opportunity – such as a compromised immune system. HIV targets CD4+ cells, in other words, cells that bear the CD4 glycoprotein on their membrane, such as regulatory and helper T cells, monocytes, macrophages, and dendritic cells. CD4 is a co-receptor of the T cell receptor, allowing T cells to communicate with antigen-presenting cells, such as dendritic cells. In order to understand how HIV impacts the immune system, we must first understand the difference between innate and adaptive immunity. The innate immune system consists of generalized barriers to pathogenic invasion. This includes physical barriers like your skin, cell-destroying stomach acid, the inflammatory response, and cells like macrophages, neutrophils and mast cells that recognize molecules common to pathogens. Meanwhile, the adaptive immune system brings out pathogen-specific weaponry. This response is thanks to B and T cells, which recognize antigens – bits of a pathogen that allow for its recognition. When dendritic cells, a type of antigen-presenting cell and the bridge between the innate and adaptive immune systems, enter lymph nodes, B cells produce antibodies to bind the specific invader, marking it for destruction by other cells. There are four kinds of T-cells and the relevant ones here are cytotoxic and helper T-cells. Cytotoxic T-cells bind and lyse the pathogen. Helper T-cells are necessary for the activation of both cytotoxic T-cells, and B-cells, so without them, the adaptive immune response can’t be activated. Antibodies don’t get produced and infected cells aren’t lysed. Unfortunately, helper T-cells are CD4+, and the main target of HIV. As the infection progresses, there is a substantial loss of these cells. The immune system fights back by producing more T-cells, some of which differentiate into helper T-cells. This fight for supremacy continues for years, until the virus eventually tips the scales. The body loses its ability to produce T-cells, and even the weakest pathogens become problematic . 3D models from: https://www.turbosquid.com/FullPrevie... https://www.cgtrader.com/3d-models/ch... HIV graph based on: https://en.wikipedia.org/wiki/HIV#/me...