• General information on human immune system
  
  
  • T-cell binding
  
  
  
• Immune responses
  
  
  • Human immune system map
  
  
  
  • CD binding
  
  
  
  • Interleukin propagation
  
  
  
  • Perforin lysing
    

In human immune system, there are two major types of lymphocytes, B and T lymphocytes. Concentrated in the spleen and lymph nodes, lymphocytes circulate the body through the circulatory system. Because lymphocytes are induced only by certain antigens, they are characterized by specificity.

	Figure 1: Antibodyand its specific antigen-binding sites. This demonstrates that antibody is specific when interacting with an antigen.
Courtesy of Biology, 5th edition

Both B and T cells recognize specific antigens by their antigen receptors. While receptors on B cells form in a secreted form, T-cell receptors are never created in such way. Typical lymphocyte contains approximately 100,000 receptors, which are produced during random DNA rearrangement in the maturation of an immunoglobulin gene. T cells are crucial when interacting with a family of genetic codes termed the major histocompatibility complex (MHC). There are two types of MHC: Class I MHC and Class II MHC. The function of MHC molecules is to present antigen fragments to T cell receptors. While Receptors of cytotoxic T cells (Tc) bind to the class I MHC molecules, those of helper T cells (TH) bind to antigens of class II MHC molecules [T cell binding].

The human immune system can be divided into two major sections: humoral and cell-mediated immunity responses. The humoral immunity response usually defends against free bacteria, toxins, and viruses. The cell-mediated immunity response, on the other hand, is active against bacteria and viruses within infected cells. After antigens have been destroyed, memory B, cytotoxic T, and helper T cells are formed.

Helper T lymphocytes: When B cells or macrophages engulf antigens, they become antigen-presenting cells (APCs), informing helper T cells that foreign materials are present. When helper T cells bind to APCs, the interaction is enhanced by a cell surface protein termed CD4, which has a molecular affinity for class II MHC proteins [CD binding]. Once activated, helper T cells release numerous different cytokines: interleukin-2 (IL-2) and interleukin-1 (IL-1). IL-2 causes B cells in vicinity to differentiate into antibody-secreting plasma cells and activates cytotoxic T cells. IL-1 activates helper T cells to produce IL-2 and other cytokines, inducing a positive feedback of the immune response [interleukin propagation]. Helper T cells, in fact, trigger both humoral and cell-mediated responses. After antigens have been removed from the body system, suppressor T cells (Ts) are formed to terminate the immune responses.

	Figure 2: Cytotoxic T cell destroys a foreign or cancerous cell by causing ions to flow into the target cell's inner membrane, lysing the structure.
Courtesy of Biology, 5th edition

Cytotoxic T lymphocytes: After a cytotoxic T cell has been activated by a class I MHC or a helper T cell in vicinity, it binds tightly to a class I MHC with help of CD8, a protein that has affinity for class I MHC proteins [CD binding]. After binding, it kills its target cell by releasing perforin, a protein that creates a membrane attack complex (MAC) on the cell's surface. By causing ions and water molecules to flow into the cell, MAC completely lyses the target cell [perforin lysing].