Immunity: Defense Mechanisms, Immune Response and Vaccines

1. Concept of Immunity

Immunity is the set of defense mechanisms that an individual possesses to confront invasion by any foreign body and the appearance of tumors. Any kind of immunity matures and consolidates over the years of life. In invertebrates, it involves cells that differentiate into the same agencies as the DCIR, and their types are recognized by immune cells.

The immune system is responsible for conferring immunity. This system, present in invertebrates, reaches its peak in the complexity of humans, primates, and others. The science in charge of studying these processes is called immunology.

2. Defenses of the Body Against Infection

2.1. Non-specific defenses

Present in the body naturally, they are defined as the mechanisms that tend to prevent the invasion of microorganisms. They are of two types: those that prevent the agent from entering, and those that combat the invader once it has entered.

2.1.1. External innate mechanisms

a) Physical barrier. The skin of animals, thanks to the stratum corneum, which suffers continuous desquamation, prevents colonies of microorganisms from proliferating and penetrating. Only some spirochetes can cross the membranes despite the barrier's effect.

b) Chemical barrier. The natural orifices are lined with mucous secretions in order to trap foreign particles and enable their expulsion. Mucus substances have the ability to deceive certain viruses, causing them to believe they have entered a cell; once inside, the virus sheds its nucleic acid, losing the ability to infect the cells.

• The presence of fluids in certain areas, for example tears in the eyes and saliva in the mouth, wash and drag microorganisms away, preventing them from installing themselves.
• These fluids contain antimicrobial substances; for example, saliva contains lysozyme, semen, spermine, etc. As a curiosity, this may explain why frequent ocular infections occur in men and in women.
• Some substances secreted by the skin modify conditions that hinder the survival of germs. One example is the stomach, which has no digestive secretion but secretes antimicrobial substances or fatty skin acids such as lactic acid.

c) Native flora. Many microorganisms naturally live in certain parts of the body. For example, bacteria that form the intestinal flora prevent others from installing themselves by secreting substances or by competing for nutrients.

2.1.2. Internal innate mechanisms

Once the foreign agent has overcome the previous obstacles, internal mechanisms intervene, involving both cellular and acellular responses.

• a) Natural killer cells (NK). These are lymphoid cells, a type of lymphocyte, that kill microorganisms, infected cells, tumor cells, or other abnormal cells. They do not recognize them as self. They destroy their targets by releasing perforin, a protein that forms holes in the membrane of the attacked cells, killing them. These are known as killer cells.
• b) Interferon. These are naturally occurring protein molecules secreted by virus-infected cells. Adjacent cells that capture them are stimulated to synthesize antiviral enzymes, which prevent viral proliferation by inhibiting the replication of the viral genome, inhibiting protein synthesis, or activating NK cells to destroy infected cells.
• c) Complement. It is formed by macromolecular complexes of proteins synthesized in the liver and circulating in the blood. When activated by various substances or antibodies, polysaccharides, for example, it originates a series of chain reactions that can produce different defensive actions. For example, some of these substances can cause cells to lyse by forming holes in their membranes, emptying the cell and killing it. Other complement proteins bind to bacteria and induce surface binding of other complement molecules that attract phagocytes, which will engulf the complement-coated microorganisms.
• d) The inflammatory response. This is part of innate immunity and occurs when tissues are injured by bacteria, trauma, toxins, heat, or any other cause. Chemicals, including histamine, bradykinin, and serotonin, are released into the injured tissue, causing blood vessels to leak fluid into the tissue, which drives localized inflammation. This helps isolate the foreign substance and limit its contact with other tissues.

2.2. Specific defenses

Over evolutionary time, microorganisms and cellular parasites that attack us, including cells such as macrophages, have developed countermeasures. Innate responses are not always effective against them, so specific defenses have developed. These defenses evolved as a counterweight to the non-specific mechanisms. While non-specific mechanisms are always present, specific defenses are activated only when a particular foreign agent invades.

These responses are cellular and humoral: lymphocytes and antibodies. The characteristic of this system is that it specifically recognizes parasites, transplanted bodies, cancer cells, microorganisms, and toxic substances produced by them. The immune system of an individual is capable of responding to self and non-self. During the development of the immune system, the body learns to recognize what is proper, and this ability to recognize self is called immunological tolerance. When tolerance is lost, autoimmune diseases appear.

Hypersensitivity reactions may also occur, such as allergies, which are immune responses to substances that are in principle harmless (for example, pollen). Foreign cells and substances that the body does not recognize as its own, against which the immune response is directed, are called antigens. Almost any macromolecule (proteins or polysaccharides, in particular) can trigger an immune response if it is recognized as foreign by the receptors.

Lymph nodes act as filters in the circulation, trapping microbes, foreign particles, and dead cells from tissues. They contain lymphocytes and macrophages, and within them the interactions responsible for the immune response occur.

4. The Immune Response

When the agents to which the body has acquired immunity are encountered, the response triggers two types of reaction:

• The humoral immune response: The objective of this response is the production of antibodies by plasma cells. These bind to foreign agents and molecules that carry antigenic capacity, causing a series of chain reactions that lead to their destruction and to their phagocytosis, fundamentally by macrophages. This response is addressed primarily to foreign agents such as bacteria and viruses that circulate outside infected cells, preventing them from infecting new cells.
• The cellular immune response: The humoral response is ineffective when the target is inside the body's own cells. The cellular response is aimed at destroying infected cells and preventing the agents from continuing to reproduce.

Both responses act in a coordinated way against circulating pathogens, and against agents that are inside cells or toxins produced by them.

5. Immunization: Vaccines and Serums

5.1. Vaccines

Vaccines are antigenic preparations made from virulent microorganisms, either dead or attenuated, or from molecules that lack toxicity. They are obtained from microorganisms or other infectious agents and induce acquired active immunity against those agents, being inoculated with a minimum of local and general reactions. Vaccines must have two properties:

• Efficiency: they must trigger a correct immune response.
• Safety: the vaccine must be free of pathogenicity, and its objective must not be to interfere with the immune response.

5.2. Serums

Serums provide immediate immunity by means of biological preparations that contain the specific antibodies needed. They are a fast intervention, though short-lived, and are more intense than vaccination. Serums do not participate in the production of molecules and confer passive acquired immunity. There are two types of serum:

• Homologous serums: obtained from humans who have antibodies against a given antigen.
• Heterologous serums: produced from other species, containing antibodies against a specific antigen.

6. Immunopathology

6.1. Description of autoimmune diseases: concept and types

The immune system, including lymphocytes, macrophages, and other cells, is tolerant to every cell and organism of the body and does not attack its own proteins, only foreign invaders. However, immature lymphocytes can sometimes react against the body's own antigens. Normally, immune cells are prevented from doing so while they mature in the thymus or bone marrow, where they are destroyed or inactivated when they recognize the body's own molecules. However, by some mechanism these safety controls can fail, and lymphocytes can escape inactivation or destruction and trigger an immune response against the body's own molecules or cells, giving rise to autoimmune disease.

Autoimmune diseases can affect any organ, and some are more frequent than others.

6.5. Rejection of transplants

Since ancient times, transplants have been used to solve technical situations where the health or life of an individual is in danger. In transplants, the tissue or damaged organ is removed and replaced with a healthy one from another individual, and the success of the recipient's survival depends on several conditions. In the case of autotransplants, where a tissue is moved from one position to another within the same body, the procedure always succeeds if surgical and aseptic techniques are used. Transplants can also succeed between genetically identical donors and recipients, such as identical twins. Alternatively, transplants can be made between genetically different individuals of the same species. Transplants between different species, known as xenotransplants, are also performed. In the last two cases, the transplanted tissue will generate, on the part of the recipient, a destructive immune response known as rejection. It originates in the existence of proteins on the surface of cell membranes (molecules such as MHC); if these foreign molecules are recognized, a specific immune response is triggered. In order to avoid these problems, immunologists perform histocompatibility tests before transplants.