The 4 types of hypersensitivity, and their characteristics

The immune system is essential for the maintenance over time of complex organisms, as it reacts and eliminates invaders such as bacteria, parasites, viruses, foreign substances, cancer cells and triggers inflammatory responses to end threats. In general, immune reactions prevent the onset of diseases and contribute to the healing of the individual, but on other occasions they target non-hazardous substances and cause serious tissue damage variable.

We are facing the well-known allergies or, more specifically, hypersensitivity reactions. This "immune imbalance" is a matter of increasing concern globally, since the World Health Organization (WHO) estimates that by the year 2050 approximately half of the world's population will suffer from some type of disorder allergic. Today, rhinitis occurs in 25% of human beings, neither more nor less.

In countries like Spain, it is observed that allergic rates in infants increase by 2% annually. This roughly translates to a 100% increase in queries every few years. All these data highlight the importance of immune hypersensitivity today, but do you know what these reactions consist of? If not, don't worry, because here you will know

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the 4 types of hypersensitivity.

Related article: "Leukocytes: what they are, types and functions in the human body"

What is hypersensitivity?

First of all, it is necessary to note that disorders of the immune system can be divided into the following groups:

hypersensitivity states.
Autoimmunity.
Congenital or acquired deficiency states.

It is in the first variant that we are going to focus. From a classical point of view, it is defined as hypersensitivity to an exaggerated immune reaction that produces a pathological picture causing disorder, discomfort and, rarely, sudden death.

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

Types of hypersensitivity

Hypersensitivity can be distinguished into 4 types, depending on the components of the immune system that cause the response.. We will 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. Go for it.

1. Immediate hypersensitivity

To understand the entire terminological conglomerate that we are going to launch in the following lines, it is necessary to establish certain knowledge. For example, it is essential to know that an antibody or immunoglobulin (Ig) is a type of protein circulating in the blood that "marks" potential pathogens by binding to their antigen (Ag). The antibodies have a typical Y shape in their monomeric variant, with a variable (Fab) and a constant (Fc) fraction. The immune cells in charge of eliminating the pathogen adhere to the Fc area 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 certain antigens on the membranes of these cells.

After a period of sensitization (a first contact with the allergen), the cells "put on alert" secrete components pharmacological such as histamine, leukotriene and prostaglandins, whose immediate effect is vasodilation and contraction of the smooth muscle. It is an immediate response, the symptoms of which largely depend on the form of entry of the antigen, the soluble dose of the antigen, and the type of response of the mast cells. These types of reactions cause 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 skin reactions.
Low blood pressure (Hypotension).
Constriction of the airways, which can cause difficulty or inability to breathe.
Weak and fast pulse.
Nausea, vomiting and diarrhea.
Loss of consciousness and fainting.

In the event of a systemic allergic response, the only possible option is to go promptly to an emergency center or, if defect, perform the epinephrine injection on the patient that he should carry with him and then call the professionals doctors. In this serious clinical picture, every second counts.

You may be interested in: "The 13 types of allergies, their characteristics and symptoms"

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 cells themselves of the patient.

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

A clear example is autoimmune hemolytic anemia. In it, antibodies are generated against 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.

Even so, there are many other pathologies mediated by cytotoxic antibodies. Among them we can find Goodpasture 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 others pathologies. Remember this: In this variant, the antibodies bind to cells that they shouldn't, causing their early destruction. It has nothing to do with the typical allergic response.

You may be interested in: "Main Cell Types of the Human Body"

3. Immune complex-mediated hypersensitivity

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

Unfortunately, when the aggregations of IgM and IgG immunoglobulins with their antigens are too too large to be removed, can be deposited in tissues and lead to signals of immune attack wrong. On the other hand, if the dose of antigens is very high and given intravenously, more immune complexes are produced than can be eliminated by the body, so they accumulate inside the vessels, kidneys and joints. The most common symptoms in these cases are vasculitis, nephritis, and arthritis, which only appear sporadically until the immune complexes are completely eliminated.

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

4. delayed hypersensitivity

Also known as "cell-mediated", this type of hypersensitivity, as its name suggests, is mediated by T lymphocytes. These lymphocytes become sensitized when they come into contact with a specific antigen, and can damage tissue through its direct toxic effect or through the release of soluble substances (lymphokines). In summary, they are late responses that occur before an antigen to which the lymphocytes were already sensitized.

You may be interested in: "High lymphocytes: causes, symptoms and treatment"

final considerations

As you may have seen, when we talk about allergies we are referring to immediate hypersensitivity, mediated by IgE immunoglobulins. The rest are not allergic processes per se, because it is not that the immune system is acting in a disproportionately due to a foreign agent, but rather destroys the body's own tissues by mistake. Without a doubt, types of hypersensitivity 2, 3 and 4 are much more harmful and much less common than the first (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 subject to error, being able to act excessively against substances that are not really harmful. and even killing essential cell components for the body itself.

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

Bibliographic references:

Anaphylaxis, Mayoclinic. Collected on January 4 in https://www.mayoclinic.org/es-es/diseases-conditions/anaphylaxis/symptoms-causes/syc-20351468#:~:text=La%20anafilaxia%20es%20una%20reacci%C3%B3n, the%20sting%20of%20a%20bee.
Immune Hemolytic Anemia, Medlineplus.gov. Collected on January 4 in https://medlineplus.gov/spanish/ency/article/000576.htm#:~:text=La%20anemia%20hemol%C3%ADtica%20inmunitaria%20ocurre, to%20these%20gl%C3%B3bules%20as%20extra%C3%B1os.
Garcia Tamayo, F. (1981). The perspectives of immunological damage. Bowl. med. Hosp. Infant. Mex, 865-72.
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