**Understanding How the Immune System Fights Infection**Johnny has just been infected and needs your help in letting him know how his body is fighting the infection. To do so, describe how the innate and adaptive immune systems work together. You should address the following:1. **Define Adaptive and Innate Immunity**: - Innate immunity serves as the first line of defense and responds quickly to pathogens using general mechanisms. - Adaptive immunity involves a slower response but targets specific pathogens with precision, developing memory for future encounters.2. **Describe the Role of Natural Killer Cells**: - Natural killer cells are part of the innate immune system, identifying and destroying infected or cancerous cells by recognizing changes on the surface of these cells.3. **What are Antigen Presenting Cells and Their Function, What are MHCs**: - Antigen presenting cells (APCs) present antigens to T cells, activating the adaptive immune response. Major Histocompatibility Complexes (MHCs) are molecules on cell surfaces that display antigens.4. **Describe the Role of the Complement System**: - The complement system enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells, promotes inflammation, and attacks the pathogen's plasma membrane.5. **What is the Role of Interferons**: - Interferons are proteins produced in response to pathogens, particularly viruses, and help inhibit viral replication, activating immune cells.6. **How Do Helper T Cells, Cytotoxic T Cells, and B Lymphocytes Work**: - Helper T cells activate other immune cells. Cytotoxic T cells kill infected or cancerous cells. B lymphocytes produce antibodies as part of the adaptive immune memory.7. **Define HIV, Which Cells Do They Attack and Why Some Individuals Develop Persistent Lymphadenopathy**: - HIV infects and destroys helper T cells, leading to a weakened immune system. Persistent lymphadenopathy, or swollen lymph nodes, can occur as the body tries to fight the infection.This text introduces the basic principles of how the immune system operates to protect the body from infections, explaining the roles of different cells and processes involved.

The main function of immune system is to distinguish between self and non sell. its primary function is to protect vertebrates against microorganisms such as viruses, bacteria and parasites. It continuously scans countless number of molecular units to decide which ones are foreign and initiate the destruction. the main features of immune system are specificity and memory.1. Innate immunity is the general defence of body including the following four mechanisms- A. Phagocytosis of invaded by leukocytes and tissue macrophages. B. Destruction of microorganisms swallowed with food by HCL of gastric juice and by digestive enzymes. C. Resistance of skin to invading microorganisms. D. Destruction of Invaders and toxins in the blood itself by certain defensive compounds such as lysozyme, properdin basic polypeptides and naturally occurring antibodies which are always present in the blood plasma. This innate immunity make human body resistance to certain animal disease such as hog Cholera, cattle plague, dysentery. on the other hand it makes animal body resistant to certain severe human diseases such as poliomyelitis, mumps, syphilis, human cholera etc. Adaptive immunity is the ability to develop special defence against strong Invaders and toxins which may cause serious and even fatal diseases. it is of two types- humoral and cellular immunity.In humoral immunity body develops circulating antibodies which are globulins capable of attacking invader micro-organisms or toxins. In cellular immunity, on the other hand, the body produces a large number of specific sensitized Lymphocytes which attached to and destroy specific invaders. 2. Killer cells are the part of cell mediated immunity or cellular immunity. A killer cell expresses CD8. CD stands for cluster of differentiation a cell surface marker that identifies a lineage or stage of differentiation. NK cells destroy virus-infected cells by inducing the infected cell to kill itself by undergoing apoptosis. It secretes perforin that lyze the target cell by polymerizing to form transmembrane pores 10 and nm wide. 3. Antigen-presenting cells (APCs) are a heterogeneous group of immune cells that mediate the cellular immune response by processing and presenting antigens for recognition by lymphocytes such as T cells. for example, dendritic cells, macrophages, Langerhans cells and B cells. MHC class 1 and 2 protein are the presenters of peptides to T cells. Humans expresses 6 different class 1 genes and 6 different class II genes. MHC class 1 molecules present antigens to cytotoxic T cells with CD8+ receptors whereas MHC class 2 molecules present antigens to helper T cells with CD4+ receptors.