MHC, Immunity, and Immune Response: A Comprehensive Look
Major Histocompatibility Complex (MHC)
MHC proteins are located in the plasma membrane of cells. They distinguish between foreign entities and the body’s own cells. This group of genes has high variability, making it highly improbable that two individuals will have the same set of proteins.
There are two types:
- MHC Class I: Present in all cells of the organism. Involved in antigen presentation to cytotoxic T cells (MHC Class I).
- MHC Class II: Present only in antigen-presenting cells (APCs) of the immune system.
Defenses Against Infections
The body has several lines of defense against infections:
- Skin and Mucous Membranes: A mechanical barrier that keeps out physical invaders. The low pH of mucus, tears, and vaginal discharge also provides protection.
- Cellular and Molecular Components: This includes cells (macrophages, granulocytes, NK cells) and molecules (complement proteins) present in tissues. These are activated non-specifically, resulting in a characteristic response:
Inflammation
Inflammation is a response that always occurs in the same manner, irrespective of the invading agent. It generates vasodilation and increased permeability, leading to redness, heat, swelling, and pain. This attracts leukocytes (via diapedesis and chemotaxis) and molecular mediators (histamine), concentrating defenses (complement activation, phagocytosis, and cytokine synthesis). These are the mechanisms of the inflammatory reaction.
- Adaptive Immunity: This involves lymphocytes and molecules synthesized specifically in response to an antigen. It provides memory and a specific reaction against the particular agent, on two fronts: humoral and cellular.
The Immune Response: Primary and Secondary
Primary Immune Response
When a pathogen invades the body, it causes inflammation in the area of invasion, initially playing a defensive role.
Antigens are quickly detected by B and T lymphocytes, which are activated and begin the specific immune response, both humoral and cellular.
Activation of TH1 Cells
The antigen must be phagocytosed by an APC. Fragments of microbial proteins are degraded and exposed in a groove of the MHC Class II protein. In this way, the APC presents the antigen to activated TH1 lymphocytes, causing the production of effector T cells and memory cells. These stimulate TH1 cells themselves, macrophages, and cytotoxic T lymphocytes (TC).
Activation of TC Cells
If microbes have invaded cells, they manufacture some aspects that appear on the cell surface. These proteins associated with MHC Class I complexes are recognized by activated TC lymphocytes, which destroy the infected cells. Activated TC lymphocytes secrete cytokines that amplify the immune response.
Activation of B Lymphocytes
Most antigens are unable to stimulate B cells without the help of TH2 cells, which, when activated, cause B lymphocyte proliferation and differentiation into plasma cells that secrete antibodies.
Secondary Immune Response
When a pathogen enters the body a second time, it generates a secondary immune response that is faster, more intense, and more effective. Although the mechanisms are similar, it relies on memory cells.
Natural Immunity
A person who has had a disease develops natural active immunization. Fetuses and newborns undergo passive immunization through natural antibodies received from the mother’s placenta or breast milk.