Why do pathogens have antigens

1: Basics of defense against infection

The immune defense has three tasks: locate, identify and destroy intruders. Once a pathogen has overcome protective barriers such as the skin or the mucous membrane, it encounters specialized defense actors.


Image with caption
Scavenger cell (model)

The patrols of the immune system: phagocytes = macrophages

The scavenger cells belong to the front fighters of the immune defense. They eat pretty much anything that seems strange to them. Since they attack all possible pathogens in an untargeted way, they are among the representatives of non-specific defense. Especially in the early phase of an infection, the phagocytes ensure that bacteria, viruses or fungi are kept in check. If the unspecific defense succeeds in eliminating all pathogens, the infection is defeated. No immunological memory is formed.



Image with caption
Animated film "The unspecific defense"

If, on the other hand, the unspecific defense cannot cope with the invasion of pathogens, the phagocytes alert other helpers. They break down the pathogen and present fragments of it like a wanted poster on their cell surface. Such a profile, for example a piece of the outer shell of a bacterium, is also known as an antigen.


The alarm system of the phagocytes

Scavenger cells use a special technique to show antigens on their surface. They use type-I and MHC-II transport molecules for this.


The abbreviation MHC stands for Major Histocompatibility Complex. All cells in the body have MHC-I molecules on their surface. The MHC molecules serve, so to speak, as identity cards. They differ from individual to individual and help the immune system to differentiate between foreign and self. MHC-II molecules are only found on antigen-presenting cells.


MHC molecules have a kind of recess in which the antigen is presented as if on a presentation plate. This is the only way to ensure that other cells of the immune system become aware of the pathogen and initiate appropriate defense measures. Phagocytes also get support by sending out messenger substances.


The scouts and commanders of the immune system: T-helper cells

The immune system has a huge army of scouts, the T helper cells. You can identify antigens, i.e. typical characteristics of pathogens. And as "Commander" you coordinate important parts of the defense in order to avert an infection. T helper cells are part of the specific immune defense.


Antigens can take any imaginable form. So that none of the antigens are overlooked, there is a suitable T helper cell for every possible antigen. So there is a strict division of labor: each T helper cell only ever recognizes one specific antigen.


The recognition mechanism of the T helper cell

Every T helper cell carries many receptors of the same type all around. With this, the T helper cell scans the surfaces of other cells and determines whether there is exactly the antigen that its receptor fits on.


Image with caption
T helper cell (model)

The T helper cell receptor essentially consists of two sub-receptors, the antigen receptor and the so-called 4 receptor.


  • The antigen receptor matches a special antigen.
  • The -4 receptor fits on the MHC-II molecule.

A T helper cell only recognizes an antigen if both sub-receptors can dock onto a phagocyte or another antigen-presenting cell at the same time. If only the antigen receptor fits on the surface antigen of any cell, the T helper cell is inactivated. T-helper cells can only react to antigens that are shown to them by the body's own antigen-presenting cells. The MHC-II molecule serves, so to speak, as a pass.


The activation of a T helper cell

In order for a T helper cell to initiate defense measures against a pathogen, it must first be activated by a phagocyte. This uses the costimulator, a molecular "ignition key" that starts the T-cell.


In order for a powerful immune defense to come about, the scavenger cell releases messenger substances. The T-helper cells then divide several times, and an armada of the same T-helper cells is created. They swarm out to mobilize more helpers in the fight against the pathogen.


Image with caption
B-cell (model)

The arms factories of the immune system: B cells

The T helper cells search specifically for B cells that can use the MHC-II molecule to present the antigen they are looking for and produce suitable antibodies against it. If a T helper cell can dock with its receptor, it releases messenger substances. The B-cell multiplies and "arms factories" arise that produce large quantities of antibodies.



Image with caption
Animated film "The coordination of defense"

The immune system has a large number of different B cells so that suitable antibodies can be provided for each antigen. Here, too, there is a strict division of labor: each B cell can only produce one type of antibody. B cells are part of the specific defense system.


Image with caption
Antibody (model)

The guided weapons of the immune system: antibodies

Antibodies are Y-shaped. The two arms carry identical receptors that exactly match a special antigen. Antibodies are part of the specific humoral defense.



The antibodies search for pathogens that show the corresponding antigen on their surface and attach themselves there. This has two effects:


  • The pathogens enveloped by antibodies (= antigen-antibody complexes) become more immobile and can no longer endanger the body's own cells.
  • The pathogens are marked as hostile by the antibodies and can be eliminated by phagocytes.

Antibodies differ in their receptors, but their "stems" are all the same (Fc region). Phagocytes recognize the "stalks", grab them and devour the pathogen together with the antibodies. As garbage disposal, the phagocytes gradually remove the hostile pathogens.


The immunological memory

Image with caption
Animated film "The specific defense"

When the immune system has successfully fought an infection, most of the comrades-in-arms (T helper cells, B cells) are eliminated. But some still remain in reserve as memory cells. In the event of a renewed infection with a pathogen that has already been successfully fought, the T and B memory cells are quickly reactivated and the production of suitable antibodies can start up in a very short time. The immune system is prepared for the infection: the body is immune to the pathogen.