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The body constantly defends itself from aggressions of foreign agents (bacteria, viruses, fungi, toxins, among others) coming from the exterior, as well as from our own ones present within the body.

The first line of defense in causing the immune response is made up of the skin, stomach acid, mucosa and the cilia of the respiratory ways, the cough reflex, among others. But if a pathogen agent trespasses these barriers, entering through the air, foods or water ingested or wounds in the skin, it is attacked and destroyed by the immune defenses the body has.
The immune system has two ways of reacting to the attack of a pathogen agent: developing a non-specific immune response or a specific immune response.

Non-specific and specific response

The non-specific immune response, also known as innate immune response, is the first defensive barrier of the body because we have it at birth.
The cells in charge of carrying out this response are the phagocytes and there are two kinds, the neutrophils and macrophages.
The first are the ones that enable the blood to reach the tissues when the infection of inflammation has developed and the second ones circulate in the blood and tissues of the body and attack all the foreign elements that enter the body and the waste substances of the tissues.Both cells are characterized for activating immediately after the entry of a foreign pathogen to the body as been detected. This way for example, after suffering a wound these cells head to the affected spot, recognize the invading substance, envelop it and digest it, process known as phagocytosis.Natural killer cells also participate in this type of defense, known as natural killer (NK). They attack a wide range of tumoral elements and cells infected by some virus.
The specific response, also known as acquired immune response, develops throughout life as a consequence of the exposure of the body to germs and substances present in the environment that have overcome the non-specific immune mechanisms. It is effective when facing those antigens in which the immune response has already begun and developed. In it, cells called lymphocytes (type of white blood cell responsible for immune responses) participate that can be one of two types: B lymphocytes (or B cells, that originate and mature in the bone marrow, produce antibodies and help fight infections) and T lymphocytes (or T cells, that develop in the thymus and attack the cells infected by a virus). These last ones can be helper T Lymphocytes (Th), cytotoxic T lymphocytes (Tc) and suppressor/regulatory T cells (Ts). At the same time, the specific immune response can be of two kinds: humoral and cellular.

Humoral and cellular immunity

They are two types of immunity that activate if the infection persists or spreads. In humoral immunity, the cells do not directly attack the antigens, they use proteins called antibodies to fight the invaders, especially bacteria and some kinds of viruses. This response is performed by the B lymphocytes. When the antigen enters into contact with the B lymphocytes, they recognize parts of their structure in their receptor.

In response to this recognition, the B lymphocytes activate and transform into plasma cells that secrete antibodies, which will couple to the antigen to then destroy it.The cellular type immune response directly destroys the antigens. It mainly acts before bacteria and viruses as well as preventing the appearance and development of cancer cells. T lymphocytes that have a receptor for the antigens called TCR participate in it. Its function is to send a signal of recognition of the antigen to the inside of the T lymphocyte.