Immune System

Anatomy

3/20/2026

The main function of the immune system is to recognize pathogens, fight, and defend the body against foreign substances. Its purpose is to have immunity, which is the body's resistance to diseases. 

In the immune system, there are 3 lines of defense. The first line of defense is the skin, which acts as a barrier, and the mucous membranes, which catch foreign substances before they go into the body. The second line is called non-specific immunity, containing certain white blood cells like mast cells and macrophages. Mast cells recognize pathogens and respond by releasing a chemical called histamine, which recruits other cells to the danger, while macrophages eat and consume foreign invaders. This line of defense focuses on reacting the same way to everything. Finally, the last line of defense is the specific/adaptive immunity defense. This part of the immune system is specialized in specific pathogens that come from memory cells that can "remember" how to defeat an antigen the body has previously been exposed to. 

Along with mast cells and macrophages, there are 8 important white blood cells: neutrophils, basophils, eosinophils, monocytes, dendritic cells, lymphocytes, NK cells, B cells, and T cells. Neutrophils are the most abundant of the granular white blood cells, have 2-5 lobes, and are a part of the nonspecific first line of defense. Basophils are the least common of the granular white blood cells, and are a part of local inflammation, and are a part of a positive feedback loop that intensifies inflammation. Eosinophils also play a role in inflammation, but mostly focus on allergic responses and parasites such as parasitic worms. Monocytes are the "first responders", engulf and digest pathogens, and have two subcategories. The first is dendritic cells that "catch" antigens and deliver them to T cells. The other are macrophages. Lymphocytes are the cells that produce antibodies and focus on long-term immunity. In lymphocytes, there are three subcategories. NK cells destroy harmful cells, whether infected or malignant, and release perforin, which induces apoptosis in those cells. B cells also produce antibodies, paralyze pathogens, stop them from spreading, and sterilize them. Finally, T cells have two different types: Helper T, which identify antigens, and Killer T, which kill infected cells by targeting specific enzymes in the cells.