Immune system
![A scanning electron microscope image of a single neutrophil (yellow), engulfing anthrax bacteria (orange)](/Images/godic/202502/09/Neutrophil_with_anthrax_copy0416.jpg")
![A scanning electron microscope image of normal circulating human blood. One can see red blood cells, several knobby white blood cells including lymphocytes, a monocyte, a neutrophil, and many small disc-shaped platelets.](/Images/godic/202502/09/SEM_blood_cells0416.jpg")
![Function of T helper cells: Antigen-presenting cells (APCs) present antigen on their Class II MHC molecules (MHC2). Helper T cells recognize these, with the help of their expression of CD4 co-receptor (CD4+). The activation of a resting helper T cell causes it to release cytokines and other stimulatory signals (green arrows) that stimulate the activity of macrophages, killer T cells and B cells, the latter producing antibodies. The stimulation of B cells and macrophages succeeds a proliferation of T helper cells.](/Images/godic/202502/09/Lymphocyte_activation_simple0416.png")
![An antibody is made up of two heavy chains and two light chains. The unique variable region allows an antibody to recognize its matching antigen.[58]](/Images/godic/202502/09/Antibody0416.JPG")
The immune system is a system of many biological structures and processes within an organism that protects against disease. To function properly, an immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue. In many species, the immune system can be classified into subsystems, such as the innate immune system versus the adaptive immune system, or humoral immunity versus cell-mediated immunity. In humans, the blood–brain barrier, blood–cerebrospinal fluid barrier, and similar fluid–brain barriers separate the peripheral immune system from the neuroimmune system which protects the brain.