4 types of hypersensitivity, and their characteristics

Summary of the types of hypersensitivity that can affect us, and their associated pathologies.

The immune system is essential for the maintenance of complex organisms over time, as it reacts to and eliminates invaders such as bacteria, parasites, viruses and viruses.It reacts and eliminates invaders such as bacteria, parasites, viruses, foreign substances, cancer cells and triggers inflammatory responses to eliminate threats. In general, immune reactions prevent the onset of diseases and contribute to the healing of the individual, but at other times they are directed against non-hazardous substances and cause tissue damage of varying severity.

These are known as allergies or, more specifically, hypersensitivity reactions. This "immune mismatch" is a matter of increasing global concern, as the World Health Organization (WHO) estimates that by the year 2050 approximately half of the world's population will suffer from some form of allergic disorder. As of today, rhinitis is present in 25% of human beings, no more and no less.

In countries such as Spain, Allergy rates in infants are increasing by 2% annually. This translates, approximately, into a 100% increase in consultations every few years. All these data highlight the importance of immune hypersensitivity nowadays, but do you know what these reactions consist of? If you do not, do not worry, because here you will learn about the 4 types of hypersensitivity.

What is hypersensitivity?

First of all, it is necessary to point out that alterations of the immune system can be divided into the following groups:

• Hypersensitivity states.
• Autoimmunity.
• States of congenital or acquired deficiency.

It is on the first variant that we are going to focus. From a classical point of view, hypersensitivity is defined as an exaggerated immune reaction that produces a pathological condition causing disorders, discomfort and, in rare cases, sudden death..

This event has much in common with autoimmunity, since it is defined from a clinical point of view (according to medical specialists) as an excessive or inadequate immune response to environmental antigens, usually non-pathogenic, causing tissue inflammation and organ malfunction.

Types of hypersensitivity

Hypersensitivity can be distinguished in 4 types, depending on the components of the immune system that originate the response. We tell you about each of these mechanisms below, following the Gell and Coombs scale, the most famous of all at the clinical and epidemiological level. Let's get down to it.

Immediate hypersensitivity

In order to understand the whole terminological conglomerate that we are going to throw at you in the following lines, it is necessary to have some basic knowledge. For example, it is essential to know that an antibody or immunoglobulin (Ig) is a type of protein circulating in the blood, which "marks" possible pathogens by binding to their antigen (Ag). Antibodies have a typical Y-shape in their monomeric variant, with a variable fraction (Fab) and a constant fraction (Fc). The immune cells in charge of eliminating the pathogen attach to the Fc site of the antibody, which in turn is bound to the antigen.

Good. Having commented on this, we can say that, in immediate hypersensitivity, basophilic leukocytes and mast cells bind IgE antibodies to specific antigens on the membranes of these cells..

After a period of sensitization (a first contact with the allergen), the "alerted" cells secrete pharmacological compounds such as histamine, leukotriene and prostaglandins, the immediate effect of which is vasodilatation and smooth muscle contraction. This is an immediate response, the symptomatology of which is quite dependent on the form of antigen entry, the soluble dose of the antigen and the type of mast cell response. This type of reaction causes atopy, anaphylaxis and asthma.

The severity of this clinical picture depends on whether the response is localized or systemic.. When the allergic response is systemic and severe, we are dealing with a case of anaphylaxis, characterized by the following symptoms:

• Sudden cutaneous reactions.
• Low blood pressure (hypotension).
• Constriction of the airways, which may cause difficulty or impossibility to breathe.
• Weak and rapid pulse.
• Nausea, vomiting and diarrhea.
• Loss of consciousness and fainting.

In the face of a systemic allergic response, the only possible option is to rush to an emergency center or, failing that, to give the patient the epinephrine injection that he should carry with him and then call the medical professionals. In this serious clinical condition, every second counts.

2. Cytotoxic antibody hypersensitivity.

Also known as plain antibody hypersensitivity, this variant is characterized by the binding of antibodies produced by the immune system to antigens present on the surfaces of the patient's own cells. the patient's own cells.

In this case, immunoglobulins (or antibodies, it is the same thing) IgM and IgG are involved. These cells, which appear to be pathogenic but in fact are not, are recognized by macrophages and dendritic cells, which act as antigen presenters, encouraging B lymphocytes to generate even more antibodies for them. Thus, cells that really are not pathogenic end up being identified as pathogenic, with their consequent erroneous destruction.

A clear example is autoimmune hemolytic anemia. In this case, antibodies are generated against the circulating erythrocytes or red blood cells, which end up being destroyed and cause pathologies in the patient. As you will see, this is not an allergic response, but a failure of the immune system.

However, there are many other pathologies mediated by cytotoxic antibodies. Among them we can find Goodpasture's syndrome (the immune system attacks the glomeruli of the kidney and the tissue of the pulmonary alveoli), pemphigus (destruction of epidermal structures), immune thrombocytopenia (erroneous destruction of circulating platelets), rheumatic fever and many other pathologies. Remember the following: in this variant, antibodies bind to cells they should not, causing their early destruction. It has nothing to do with the typical allergic response.

3. Immunocomplex-mediated hypersensitivity

This type of hypersensitivity is caused by the deposition of immunocomplexes in certain tissues.. We know as immunocomplexes the union of an antigen and an antibody (Ag-Ac), which are normally eliminated during the development of the immune response.

Unfortunately, when aggregations of IgM and IgG immunoglobulins with their antigens are too large to be eliminated, they can be deposited in tissues and lead to erroneous immune attack signals. On the other hand, if the dose of antigens is very high and intravenous, more immunocomplexes are produced than can be eliminated by the organism, so that they accumulate in the tissues.The most common symptoms in these cases are the following. The most common symptoms in these cases are vasculitis, nephritis and arthritis, which only appear sporadically until the immunocomplexes are completely eliminated.

Other pathologies related to this type of hypersensitivity are glomerulonephritis (inflammation of the kidney glomeruli), rheumatoid arthritis, subacute bacterial endocarditis (inflammation of the cardiac tissue) and systemic lupus erythematosus, among others.

4. Delayed hypersensitivity

Also known as "cell-mediated", this type of hypersensitivity, as the name suggests, is mediated by T-lymphocytes, is mediated by T-lymphocyte-mediated T-cells.. These lymphocytes become sensitized upon contact with a particular antigen, and can damage the tissue by their direct toxic effect or through the release of soluble substances (lymphokines). In short, they are late responses to an antigen to which the lymphocytes were already sensitized.

Final considerations

As you may have noticed, when we talk about allergies we are referring to immediate hypersensitivity, mediated by IgE immunoglobulins. The rest are not allergic processes per se, since it is not that the immune system is acting disproportionately due to a foreign agent, but that it destroys the body's own tissues by mistake. Without any doubt, hypersensitivity types 2, 3 and 4 are much more pernicious and much less common than the first one (unless we are talking about anaphylaxis).

Summary

As we have said in the first lines, the immune system is essential for the well-being and maintenance of the organism. Even so, like all living machinery, it is like all living machinery, it is subject to error, and can act excessively against substances that are not really harmful and even kill cellular components of the body. and even killing cellular components essential to the organism itself.

Imagine the disaster of the immune system conceiving red blood cells or platelets as a threat. All this translates into a cascade effect that manifests with multiple symptoms in the patient, most of them severe. Fortunately, these conditions are not common.

Bibliographic references:

• Anaphylaxis, Mayoclinic. Retrieved January 4 from https://www.mayoclinic.org/es-es/diseases-conditions/anaphylaxis/symptoms-causes/syc-20351468#:~:text=The%20anaphylaxis%20is%20a%20reaction%20reaction%20to%20the%20sting%20of%20a%20bee.
• Immune hemolytic anemia, Medlineplus.gov. Retrieved January 4 from https://medlineplus.gov/spanish/ency/article/000576.htm#:~:text=The%20anemia%20hemol%20hemol%C3%ADtica%20immune%20occurs,a%20these%20gl%20gl%C3%B3bules%20as%20extra%20extra%20C3%B1os.
• Garcia Tamayo, F. (1981). The prospects of immunologic damage. Bol. med. hosp. infant. Mex, 865-72.
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