Medulla oblongata at the level of olive

Medulla oblongata at the level of olive

he medulla oblongata (medulla) is one of the three regions that make up the brainstem. It is the most inferior of the three and is continuous above with the pons and below with the spinal cord. The medulla houses essential ascending and descending nerve tracts as well as brainstem nuclei.

In this article, we shall look at the anatomy of the medulla – its external features, internal anatomy, and blood supply.

External Anatomy of the Medulla

The medulla is conical in shape, decreasing in width as it extends inferiorly. It is approximately 3cm long and 2cm wide at its largest point.

The superior margin of the medulla is located at the junction between the medulla and pons, while the inferior margin is marked by the origin of the first pair of cervical spinal nerves. This occurs just as the medulla exits the skull through the foramen magnum.

Anterior Surface

There are several structures visible on the anterior surface of the medulla – namely the three fissures/sulci, the pyramids, the olives, and five cranial nerves.

In the midline of the medulla is the anterior median fissure, which is continuous along the length of the spinal cord. However, it is interrupted temporarily by the decussation of the pyramids (see below). As we move away from the midline, two sulci are visible – the ventrolateral sulcus and the posterolateral sulcus.

The pyramids are paired swellings found between the anterior median fissure and the ventrolateral sulcus. Information on the pyramids can be found here. The olives are another pair of swellings located laterally to the pyramids – between the ventrolateral and posterolateral sulci.

Arising from the junction between the pons and medulla is the abducens nerve (CN VI). Extending out of the ventrolateral sulcus is the hypoglossal nerve (CN XII). In the posteriolateral sulcus, three more cranial nerves join the medulla (CN IX, CN X, and CN XI).

Posterior Surface

Unlike the anterior surface of the medulla, the posterior surface is largely obstructed from view and is relatively devoid of features. In order to appreciate the posterior surface, the cerebellum must be removed.

Similar to the anterior surface, the posterior surface has a midline structure – the posterior median sulcus – which is continuous below as the posterior median sulcus of the spinal cord. Above, the sulcus ends at the point in which the fourth ventricle develops.

As we move lateral from the midline, the fasciculus gracilis and fasciculus cuneatus are seen, separated by the posterior intermediate sulcus.

Medulla oblongata at the level of olive

Internal Anatomy of the Medulla

The internal structures of the medulla must be viewed in cross section to understand the layout. Three levels of the medulla are typically discussed (inferior – superior):

  • Level of decussation of the pyramids
  • Level of decussation of the medial lemnisci
  • Level of the olives

The medulla itself is typically divided into two regions: the open and the closed medulla. This distinction is made based on whether the CSF-containing cavities are surrounded by the medulla (closed medulla) or not (open medulla). The medulla becomes open when the central canal opens into the fourth ventricle (see Fig. 3).

Some features are seen in all three cross sections. Anteriorly we can see the paired lumps representing the pyramids which are separated by the anterior median fissure. Centrally, the central canal can be seen as it rises to form the fourth ventricle in the final cross section.

Level of the Decussation of the Pyramids

This is the major decussation point of the descending motor fibres. Roughly 75% of motor fibres housed within the pyramids cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts.

At this level, the central portion of the medulla contains gray matter, while the outer portions consist of white matter. The posterior white matter contains the fasiculus gracilis and the more lateral fasiculus cuneatus. Corresponding portions of gray matter extend to these regions and are the nucleus gracilis and nucleus cuneatus respectively.

Unchanged from the spinal cord, the spinocerebellar tracts (posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them. The large trigeminal nucleus and tracts can be found posterior to these tracts. This is a continuation of the substantia gelatinosa of the spinal cord.

Level of Decussation of the Medial Lemniscus

This level marks the sensory decussation occurs of the medial lemniscus. (Fig. 5). Purple lines have been used to represent the internal arcuate fibres as they run from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus.

Lateral to the medial lemniscus, the trigeminal nucleus and spinal tract can once again be seen, as can the spinocerebellar tracts and the lateral spinothalamic tract. Similarly, the posterior structures are much the same at this level.

Centrally, the hypoglossal nucleus and medial longitudinal fasciculus are seen. Moving laterally, the nucleus ambiguous can be seen. Between this structure and the pyramids is the inferior olivary nucleus.

Medulla oblongata at the level of olive

Level of the Olives

This level shows significant change in structure both externally and internally when compared with previous levels. The central canal has now expanded into the fourth ventricle and as such makes this region the open medulla.

The large inferior olivary nucleus is responsible for the external expansion of the olives. The related medial and dorsal accessory olivary nuclei can be seen medial and posterior to this structure respectively.

The large inferior cerebellar peduncles come into view and are surrounded by multiple nuclei. The two vestibular nuclei (medial and inferior) are both found towards the midline while the two cochlear nuclei are found somewhat above and below the peduncles. Now a much smaller structure, the trigeminal tract and nucleus is seen adjacent to the peduncle.

The nucleus ambiguous remains as it was previously, while the hypoglossal nucleus has migrated with the central canal posteriorly, joined by the medial longitudinal fasciulus. An additional cranial nucleus comes into view lateral to the hypoglossal – the dorsal vagal nucleus. Moving further lateral, the nucleus of tractus solitarius comes into view.

Centrally, the medial lemniscus hugs the midline posterior to the pyramids, as does the tectospinal tract.

Between the peduncle and the olivary nuclei resides the lateral spinothalamic tract and the more lateral anterior spinocerebellar tract.

Vasculature

The vasculature of the medulla is complex and is dependent on the level being viewed (Fig. 7). The following attempts to simplify this complexity. Despite this it may suffice the reader to know that the vessels that supply the medulla include: the anterior spinal, the posterior spinal, the posterior inferior cerebellar, the anterior inferior cerebellar, and vertebral arteries.

Throughout the medulla, the anterior spinal artery supplies a region beginning at the central canal (or anterior border of the fourth ventricle), and fans out to encompass the pyramids.

Below the level of the olives the posterior half of the medulla is supplied by the posterior spinal artery. No other regions are supplied by this vessel. The remaining portions are supplied by the posterior inferior cerebellar and vertebral arteries.

In cross section through the olives both the posterior inferior cerebellar and vertebral arteries take on greater territories posterolaterally and anterolaterally respectively. They continue to do so as the medulla ascends.

At the highest point in the medulla, the anterior inferior cerebellar artery supplies the outermost portions of the posterior region.


Tegmental part at lower half of pons

Tegmental-part-at-lower-half-of-pons

Gray matter: Some cranial nerve nuclei and nuclei of pontine part reticular formation. l Abducent nerve nucleus: It is the nucleus of somatic efferent group. Fibers of abducent (VIth cranial) nerve arising from this nucleus supply lateral rectus muscle of eyeball which is developed from preoccipital myotome of paraaxial mesoderm. This nucleus is situated deep to a paramedian bulge adjacent to posterior median sulcus. The bulge is called facial colliculus because the surface of abducent nucleus is winded by fibers of facial nerve.

  • Motor nucleus of facial nerve: This is the nucleus of special visceral efferent column which supplies muscles developed from mesoderm of second branchial arch.efferent nucleus of facial nerve, situated lateral to motor nucleus of facial nerve. It has a component called lacrimatory nucleus. Parasympathetic secretomotor fibers from these nuclei are directed to supply to submandibular and sublingual salivary glands, and lacrimal gland.
  • Spinal nucleus of trigeminal nerve: This is exteroceptive variety of general somatic afferent nucleus of trigeminal nerve, which receives pain and temperature sensation from skin of face. Though called spinal nucleus, main part of this nucleus extends throughout whole length of medulla oblongata. Its lower end extends upto 2nd cervical segments of spinal cord and upper end extends to the lower half of pons. This nucleus is situated in the lateral part of tegmentum of lower end of pons. It receives sensory fibers of trigeminal nerve which caps dorsal aspect of the nucleus to form spinal tract of the nerve.
  • Vestibular nucleus of vestibulocochlear nerve:
    This is proprioceptive type of special somatic afferent nucleus of vestibulocochlear nerve. It is composed of superior, lateral, medial and inferior parts. Vestibular nucleus is situated partly in lower part of pons and upper part of medulla. It is placed in superficial plane at the lateral angle of pontomedullary junction. This nucleus receives afferent fibers which are nothing but vestibular fibers of VIIIth cranial nerve carrying sense of equilibrium or balance.

Tegmental-part-at-lower-half-of-pons
Efferent fibers are:

    1. Vestibulocerebellar fibers
    2. Vestibulospinal fibers
    3. Medial longitudinal bundle: Which connect vestibular nucleus with nuclei of IIIrd, IVth, VIth and XIth cranial nerves and anterior horn cells of upper cervical segments of spinal cord. It causes reflex movement of eyeball and head and neck in response to change of position body.
    4. Cochlear nucleus of vestibulocochlear nerve: It is exteroceptive type of special somatic afferent nucleus of cochlear component of vestibulocochlear nerve. It is made up of dorsal and ventral components lying dorsal and ventral to inferior cerebellar peduncle fibers at the level of pontomedullary junction.

    1. Connections of cochlear nuclei:
  • Afferent: Fibers of cochlear component of vestibulocochlear nerve carrying sense of hearing from receptors (organ of Corti) at internal ear relay in dorsal and ventral cochlear nuclei.
  • Efferent: Axons of cochlear nuclei will have to reach upto corresponding thalamic nuclei to carry impulse to sensory area of cerebral cortex. While ascending through central core of brainstem to reach the thalamus, at the level of lower end of pons, relayin a nucleus, called nucleus of trapezoid body. Before the relay, axons of both dorsal and ventral cochlear nuclei partly remain in the same side, partly cross the midline to relay in nucleus of trapezoid body of opposite side. In horizontal section, the fibers show a trapezoid shape, for which the decussating and nondecussating fibers are called trapezoid body, so the nucleus is also accordingly named.

Problems with the learning process

Problems with the learning

In some infants, perceptual processing that depends upon innate recognition may be damaged and perceptual attributes that lead to salience may not be perceived. Perception that depends upon temporal processing may be slowed and crossmodality may be impaired. Meaning may be more easily accessed visually (i.e. what is seen may make more sense than what is heard). Infants learn from the repeated familiar and respond to difference – a learning style used in the ‘habituation to repeat stimulus’ in developmental experiment.

Learning from the familiar needs repeated stimulus and is enhanced if as many features as possible are fixed and what is remembered depends upon the match between context and item. This is a stage of normal development referred to as ‘context bound’ learning, when children recognize their own cups or shoes, for example, but not ‘cupness’ or a sentence said by one person in one place but not another. Some children get stuck in this stage and require a sustained high degree of contextual or environmental sameness to show a skill. This is particularly shown in autism.

Children learn from ‘contingency’ – the event that follows within 3–5 seconds of their action. This may be disrupted by a number of mechanisms, such as:
1. failure of the adult to make the response (depressed/mentally ill caregiver);
2. the child not giving a clear enough signal (as for those who are blind or who have cerebral palsy).

Aspects of maternal or caregiver behavior that promote learning need to be sensitively adjusted to developmental level. Thus at 2 years of age, language input needs to be explicitly directive and adult actions tied to the focus of child interests. By 3 years less parental direction is needed because the child has more language and is learning to manage her own goal-setting and problem-solving skills.

Play complexity is enhanced by caregiver behaviors that maintain a child’s focus of attention rather than redirect it. Children also learn more through the process of learning itself. For example, learning particular names of shapes accelerates shape learning generally, as though attention to the ‘shape concept’ allows noticing of ‘shapeness’. The child’s ability to inhibit and select responses and to try alternatives is crucial to all cognitive learning. This is seen in the progression from the ‘trial and error’ approach, where repeatedly forcing the square into the round hole is a less useful strategy than trying alternative placements with inset puzzles, which shows more flexibility of mental skills.

The progress from sensorimotor play, from mouthing to manipulation at 6–10 months, then to imitation and ‘definition by use’ play by 12 months is followed by increasing creativity in play. Make-believe play with dolls, in which the child is reconstructing events observed, is an important element of this period. It indicates early symbolic representation and concept formation. The child begins to use language to direct or describe the action of his play, and as command of language improves the need to act out the events decreases. Lack of ideas, failure of pretence and inability to play constructively are indications of a developmental problem. The cognitive stage of mental symbolic development allows more complex thinking, including reflection and planning. Symbols (word ) facilitate thinking about, and reference to, situations that are not in the ‘here and now’. Answering simple questions dealing with nonpresent situations presents difficulty before 3 years and even primary and junior school children still tend to be concrete in their way of thinking (i.e. real objects, here and now). Early in school life, judgments are made intuitively on superficial appearances. With increasing experience and language at their disposal, children can imagine complex situations, think out the most appropriate solution and anticipate the outcome.

This requires the ability to think abstractly and imaginatively. Thus, children develop logical thinking from assimilating experience into schemes or general laws that they can apply to a range of situations. The use of symbols also helps to inhibit prepotent responses of behaviors and allows increasing distancing from the ‘here and now’ (rather like the red card in football games). Children are developing skills of representation and object substitution in the second year, but the skill of mentally comparing reality with representation (dual reality) is not clearly seen in research studies until aged 3 years. For example, children shown where an object is hidden in a scale model of a room can find it in the real room at 3 years, but not at 30 months. At 3–6 years, children get increasingly skilled at knowing that others can hold particular views, even false views, and thus have what has been called a ‘theory of mind’. In the classic Wimmer and Perner task, roughly half the 4- to 5-year-old children could correctly show ‘knowing’, whereas over 90% of 6- to 9


Cognitive and Learning Development

Learning Development

Children learn about their world by listening, observing, copying and experimenting. The world of infants is very small and their repertoire of skills limited. They learn about their world through observation, by reaching and grasping objects and by copying sounds and actions. By contrast, toddlers are mobile and their worlds are large. Their motor skills are greater and they begin to attempt constructional tasks, thereby learning about aspects such as size, shape, the properties of objects and space.
The child is an active participant in the learning process. Progress depends upon not only the learning opportunities, but also the child’s learning strategies and processes. Information processing in infants is related to later cognitive abilities in memory and speed of processing, thus in visual recognition tasks, habituation, learning, object permanence and attention, including crossmodality.

In older children, the features of new problem solving that are linked to learning are variability, ability to shift focus, frequency of self-correction and diversity of strategies.


B Cells

B Cells

B-Cell Development

B cells constitute approximately 10% of peripheral blood leukocytes. They develop in the bone marrow from hematopoietic stem cells but achieve maturity in peripheral lymphoid organs. Early progenitors committed to the B-cell lineage (pro–B cells) begin recombination at the immunoglobulin heavy-chain loci. Successful recombination leads to expression of μ–heavy chain, distinguishing them from pre–B cells. With the surrogate light chain and the Ig-α/β signaling machinery, an immunoglobulin-like heterodimer is expressed on the surface (pre–B-cell receptor). The pre–B-cell receptor signals a halt to μ–heavy-chain recombination, and Igκ or Igλ light-chain recombination begins. The surrogate light chain is replaced by a successfully formed κ or λ light chain, and the B-cell receptor is expressed as surface IgM, which distinguishes the immature B cell.

B Cells

B-Cell Responses

B cells provide humoral immunity against extracellular pathogens through the production of antibodies that neutralize pathogens and toxins, facilitate opsonization, and activate complement. Primary infection or vaccination results in prolonged production of high-affinity specific antibodies, the basis of adaptive humoral immunity. On the other hand, IgM antibodies are produced in the absence of infection, are of lower affinity, play a role in first-line defense against bacterial infection, and assist in clearance of endogenous cellular debris. Naïve follicular B cells reside in the follicles of secondary lymphoid tissues. Antigen arrives in these lymphoid organs through circulation of soluble molecules or immune complexes or via transportation by dendritic cells. The B cells, via the B-cell receptors, process the antigens in the context of MHC class II and then migrate to the T cell–B cell interface, the border between the T-cell zone and B-cell follicle, where they encounter primed TH cells of cognate specificity. This generates signals from T-cell–derived cytokines and triggers binding between CD40 ligand (CD40L, on T cells) and CD40 (on B cells) that sustains B-cell activation and promotes immunoglobulin class switching. Signaling through CD40 and its interaction with CD40 ligand on T cells is essential for the induction of isotype switching.

The effector T-cell cytokines have various functions: IL-1 and IL-2 promote B-cell activation and growth, IL-10 causes switching to IgG1 and IgG3, IL-4 and IL-13 cause switching to IgE, and TGF-β causes switching to IgA. IFN-γ, or some other undefined product of TH1 cells, appears to induce switching to IgG2. Activated B cells either migrate into the follicle and, with continued T-cell help, initiate the germinal center reaction or migrate to the marginal zone and differentiate into short-lived plasma cells. These latter cells secrete antibody for 2 to 3 weeks, which provides a rapid but transient source of effector molecules.

The B cells in the germinal center undergo specificity diversification through somatic hypermutation, and high-affinity variants are selected by survival advantage, a process termed affinity maturation. Thus within the germinal centers, sequential cycles of proliferation, B-cell receptor diversification, and selection amplify high-affinity variants of the original activated B cell. The cells that then exit the germinal center reaction give rise to the memory compartment, which consists of affinity-matured memory B cells and long-lived plasma cells. When memory cells reencounter antigen, they divide rapidly and expand their numbers or differentiate into antibody-secreting plasma cells. These long-lived plasma cells are terminally differentiated B cells incapable of further division that home to the bone marrow and secrete high-affinity class-switched antibody. These B cell responses are orchestrated with the help of T cells and their cytokines and are termed T-cell–dependent B-cell responses.


Sound Perception

Sound Perception

The ear is fully developed at birth and sound perception is possible in utero. Speech perception and recognition of voices of different speakers are present shortly after birth. The capacity for smell and touch as well as the other senses are similarly developed at birth and play an important part in the perceptual learning about the environment.


Atopic Dermatitis

Atopic dermatitis

Although atopic (infantile or flexural) dermatitis may begin at any age, it usually commences from about the sixth week onwards. It is characterized by a chronic, relapsing course. In the infantile phase lesions are present mainly on the head, face, neck, napkin area, and extensor aspects of the limbs. As the patient grows older and enters childhood, the eruption shifts to the flexural aspects of the limbs. Chronic atopic cheilitis may also be evident. Pruritus is intense and constant scratching and rubbing leads to lichenification and frequent bouts of secondary bacterial infection. Atopic eczema is commonly associated with dry skin (xerosis). Vesiculation is uncommon. there is an increased risk of dermatophyte and viral infections. The disease improves during childhood and, in over 50% of cases, clears completely by the early teens. approximately 75% of patients with atopic dermatitis have a family history of atopy and up to 50% have associated asthma or hay fever. The condition typically worsens in the winter months.

Atopic dermatitis

It is associated with an increased incidence of contact dermatitis, particularly affecting the hand. Other features that may be seen include ichthyosis (50%), nipple eczema, conjunctivitis, keratoconus, bilateral anterior cataracts, sweat-associated itching, wool intolerance, perifollicular accentuation, food intolerance and white dermatographism. 5 Infraorbital folds (DennieMorgan folds) are said to be characteristic of atopic dermatitis, particularly when double.


Psoriasis

Psoriasis

Psoriasis is a chronic relapsing and remitting disease of the skin that may affect any site. It is one of the commonest of all skin diseases, with a reported incidence of 1–2% in Caucasians. It is rare among blacks, Japanese, and native North and South american populations. Males and females are affected equally. Although psoriasis may occur at any age, it most frequently presents in the teens and in early adult life (type I psoriasis). A second peak in which the disease is often milder appears around the sixth decade (type II psoriasis).

Psoriasis

The classic cutaneous lesion of psoriasis vulgaris (plaque psoriasis), developing in about 85–90% of patients with psoriasis, is raised, sharply demarcated, with a silvery scaly surface. The underlying skin has a glossy, erythematous appearance. If the parakeratotic scales are removed with the fingernail, small droplets of blood may appear on the surface (auspitz’s sign); this is diagnostic. plaques, when multiple, are often symmetrical and annular lesions due to central clearing are a common finding. the scalp, the extensor surfaces (mainly the knees and elbows), the lower back, and around the umbilicus are particularly affected. the clinical features, however, show regional variation: scalp involvement often shows very marked plaque formation, whereas on the penis scaling is commonly minimal and the features may be mistaken for Bowen’s disease. Linear lesions (linear psoriasis) follow previous trauma (koebnerization).

Source: P. McKee, J. Calonje – McKee’s Pathology of the Skin (Elsevier)


Breast

Optimal gross examination of mammary tissue includes inspection and palpation and may be aided by x-ray examination. It is advantageous to examine tissue in the fresh state, even if frozen section is not requested, because abnormalities are more evident and tissue can be preserved for special studies, as necessary.

However, some types of carcinoma, particularly mucinous and medullary carcinoma, have smooth, rounded outlines and a soft consistency. Infiltrating lobular carcinoma may be extremely diffuse and difficult to see on gross examination, and palpation may be more helpful than visual inspection.

The number of blocks submitted from specimens varies depending on circumstances. It is important, however, that tumors be sampled thoroughly because their microscopic appearance may vary from area to area. The center and periphery of tumors should be sampled, as well as the surrounding tissue, because malignant change may be far more extensive than is suspected on gross examination. If possible, a section of the largest full-cut face of a tumor should be submitted in one block. Because noninvasive carcinoma may be an incidental finding, it is important to sample apparently normal tissue, even that around an obviously benign lesion, with concentration on the nonfatty component and, as is discussed later, areas of mammographic abnormality. The fatty component is unlikely to contain significant pathologic changes that are not also present in the nonfatty component of the specimen.


Eosinophils

Eosinophils

Eosinophils are derived from colony-forming unit–eosinophil (CFU-Eo), a progenitor that differentiates into an eosinophilic myeloblast, promyelocyte, myelocyte, and finally a mature eosinophil. Eosinophils constitute 2% to 5% of circulating leukocytes in normal individuals and are readily recognized from their prominent cytoplasmic granules, which contain toxic molecules and enzymes that are particularly active against helminths and other parasites. GM-CSF and IL-3 promote eosinophil growth and differentiation. The production of eosinophils from the bone marrow and the survival of eosinophils in peripheral tissues are enhanced by the cytokine IL-5, which maintains the viability of eosinophils through inhibition of apoptosis.

Eosinophils are prominent cells in most allergic responses. Eosinophils possess several surface markers and receptors involved in differentiation, recruitment into tissues, activation, synthesis, and release of their multiple mediators. Receptors for immunoglobulins include those for immunoglobulins G, E, and A (IgG, IgE, IgA). Eosinophils have receptors for complement components that include C1q (CR1), C3b/C4b (CR1), iC3b (CR3), C3a, and C5a. Both C3a and C5a are eosinophil chemoattractants that stimulate production of oxygen radicals by eosinophils, which potentially express several receptors for chemokines.

CCR1 is a receptor for macrophage inflammatory protein 1α, monocyte chemotactic protein 3, and the chemokine regulated on activation, normal T-cell expressed and secreted (RANTES); CCR3 is a receptor for eotaxin, eotaxin-2, eotaxin-3, monocyte chemotactic protein 3, and RANTES. Mature eosinophils, like their immature precursors, express functional heterodimeric receptors for the three cytokines— GM-CSF, IL-3, and IL-5—that promote eosinophilopoiesis and stimulate the functioning of mature eosinophils. The eosinophil’s cationic granule proteins include major basic protein (MBP), eosinophil peroxidase (EPO), eosinophil cationic protein (ECP), and eosinophil-derived neurotoxin. Another prominent protein of the eosinophil is the Charcot- Leyden crystal protein, which constitutes an estimated 7% to 10% of total cellular protein, possesses lysophospholipase activity, and forms the distinctive hexagonal bipyramidal crystals that are the hallmark of eosinophil-associated inflammation.

Eosinophils

MBP is a potent cytotoxin and helminth toxin in vitro. It can kill bacteria and many types of normal and neoplastic mammalian cells, stimulate histamine release from basophils and mast cells, activate neutrophils and platelets, and augment superoxide generation by alveolar macrophages. It can also induce bronchoconstriction and transient airway hyperreactivity when instilled into the monkey trachea. ECP, like MBP, has marked toxicity for helminth parasites, blood hemoflagellates, bacteria, and mammalian cells and tissues and has been shown in several studies to produce respiratory epithelial damage similar to that seen in severe asthma. As with MBP and ECP, EPO is highly cationic and exerts some cytotoxic effects on parasites and mammalian cells in the absence of hydrogen peroxide.

However, EPO is highly effective in combination with hydrogen peroxide and a halide cofactor (iodide, bromide, or chloride), from which EPO catalyzes the production of the toxic hypohalous acid. In the presence of these compounds, EPO is highly toxic to various unicellular, multicellular, and other targets that include viruses, mycoplasma and other bacteria, fungi, and parasites. Eosinophil-derived neurotoxin is a poor cationic toxin with only limited toxicity for helminths and mammalian cells, but it induces significant neurologic damage when injected intrathecally or intracerebrally into rabbits or guinea pigs. In allergic conditions, eosinophils may play a dual role. They can suppress the local tissue response to inflammatory mediators involved in IgE-mediated hypersensitivity reactions by inactivating histamine, platelet-activating factor, and heparin. On the other hand, eosinophils can augment destruction through the toxic effects of the products they release upon degranulation. The balance between these two seemingly contradictory functions of eosinophils in IgE-mediated reactions is still under investigation.