When pathogens such as viruses, bacteria, fungi, or parasites enter the body, the immune system launches an immune response to eliminate the infection. This response can be divided into two types: innate immunity and adaptive immunity. Innate immunity is the first line of defense against infections, which provides a rapid and nonspecific response to a wide range of pathogens. In contrast, adaptive immunity is slower but more specific and efficient in clearing the pathogen.
During an infection, the innate immune system recognizes pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs), Nod-like receptors (NLRs), and RIG-I-like receptors (RLRs). PRRs are expressed on various cells, including macrophages, dendritic cells, and epithelial cells. Upon PAMP recognition, PRRs activate intracellular signaling pathways that lead to the production of cytokines, chemokines, and type I interferons, which recruit and activate immune cells to the site of infection.
Adaptive immunity involves the activation of antigen-specific T and B cells that can recognize and eliminate the pathogen. The process of antigen presentation, which involves the recognition and processing of pathogen-derived antigens by dendritic cells and their presentation to T cells, is critical for the activation of adaptive immunity.
CD4+ T cells, also known as helper T cells, play a critical role in coordinating the adaptive immune response by producing cytokines that activate and regulate other immune cells, including CD8+ T cells, B cells, and macrophages. CD8+ T cells, also known as cytotoxic T cells, can directly kill infected cells by recognizing and destroying pathogen-infected cells displaying pathogen-derived peptides on their surface via MHC class I molecules. B cells produce antibodies that can bind to and neutralize the pathogen, or they can present pathogen-derived peptides to CD4+ T cells for their activation.
After the pathogen is cleared, a subset of T and B cells persists as memory cells that can rapidly respond to future infections with the same pathogen. This process is the basis for vaccination, which involves the administration of a harmless form of the pathogen or its antigens to stimulate an immune response and generate immunological memory.