Introduction to Macrophages - Macrophages are a type of white blood cell of the immune system. - They engulf and digest pathogens, cancer cells, microbes, cellular debris, and foreign substances. - Macrophages are involved in phagocytosis, which helps defend the body against infection and injury. - They are found in all tissues and patrol for potential pathogens. - Macrophages have various forms and names in different parts of the body.

Types and Development of Macrophages - Macrophages can be classified as M1 macrophages, M2 macrophages, and regulatory macrophages. - M1 macrophages increase inflammation and stimulate the immune system. - M2 macrophages decrease inflammation and encourage tissue repair. - Macrophages have different metabolic abilities depending on their type. - Macrophages in healthy tissues can derive from circulating monocytes or be established before birth. - Macrophages that accumulate at diseased sites typically derive from circulating monocytes. - Monocytes enter damaged tissue and become macrophages through leukocyte extravasation. - Macrophages can populate certain organs through proliferation. - Macrophages have a longer lifespan compared to neutrophils.

Functions and Role in Innate Immune Response - Macrophages are specialised in removing dying or dead cells and cellular debris. - They play a crucial role in chronic inflammation and the early stages of inflammation. - Macrophages can phagocytize neutrophils and foreign materials, such as pathogens. - When a pathogen is ingested, it becomes trapped in a phagosome and is digested by enzymes. - Macrophages are among the first cells to respond to a pathogen invasion. - Tissue resident macrophages are among the first cells to respond to a pathogen invasion. - They phagocytose incoming antigens and secrete proinflammatory cytokines. - Phagocytosis of pathogens is an important function of macrophages. - Macrophages can internalize antigens through receptor-mediated phagocytosis. - They can also secrete proinflammatory cytokines to induce inflammation and recruit other immune cells.

Macrophage Recruitment of Immune Cells - Macrophages recruit neutrophils to the infection site through chemokines such as CXCL1, CXCL2, and CXCL8. - Other immune cells recruited by macrophages include monocytes, dendritic cells, natural killer cells, basophils, eosinophils, and T cells. - Chemokines such as CCL2, CCL4, CCL5, CXCL8, CXCL9, CXCL10, and CXCL11 are involved in the recruitment of these immune cells. - Macrophages, along with dendritic cells, help activate natural killer (NK) cells through secretion of type I interferons (IFN-α and IFN-β) and IL-12. - IFN-γ produced by NK cells enhances the innate immune response by inducing a more aggressive phenotype in macrophages.

Macrophages and Adaptive Immunity - Macrophages are professional antigen presenting cells (APC) that present peptides from phagocytosed antigens on MHC II molecules. - Macrophages interact mostly with previously activated T helper cells or tissue resident memory T cells. - Macrophages provide both signals required for T helper cell activation: antigen presentation through MHC II molecules and co-stimulatory signals through CD80 and CD86 molecules. - These interactions allow T helper cells to achieve full effector function and receive survival and differentiation signals. - Macrophages in lymphoid tissues are involved in preventing antigen entry into the blood and clearing debris from apoptotic lymphocytes. - Macrophages can achieve different activation phenotypes through interactions with different subsets of T helper cells. - The two major phenotypes are classically activated macrophages (M1 macrophages) and alternatively activated macrophages (M2 macrophages). - M1 macrophages are proinflammatory, while M2 macrophages are mostly anti-inflammatory. - T helper cells of the T1 subset play a role in classical macrophage activation against intracellular pathogens. - Interaction between T1 cells and macrophages leads to the secretion of IFN-γ and upregulation of CD40 expression, activating macrophages and enhancing their ability to kill intracellular pathogens.